…ter failures

Due to various circumstances, such as a remote API unexpectedly breaking compatibility in a dependency sweeper, it can be desirable to allow the sweepers to continue after failures. This gives the operator the opportunity to try cleaning up other, unrelated infrastructure by using a new `-sweep-allow-failures` flag.

It is still up to the resource implementations to allow continuing on failures within the same sweeper, rather than exiting on the first failure encountered.

Without new flag (same previous behavior on successful sweepers):

```console
$ go test ./aws -v -sweep=us-west-2 -sweep-run=aws_dx_gateway_association_proposal -timeout 10h
2019/10/08 11:39:30 [DEBUG] Running Sweepers for region (us-west-2):
2019/10/08 11:39:30 [DEBUG] Running Sweeper (aws_dx_gateway_association_proposal) in region (us-west-2)
2019/10/08 11:39:32 Sweeper Tests ran successfully:
  - aws_dx_gateway_association_proposal
ok    github.com/terraform-providers/terraform-provider-aws/aws 3.171s
$ echo $?
0
```

Without new flag (same previous behavior on failing sweepers):

```console
$ go test ./aws -v -sweep=us-west-2 -sweep-run=aws_vpn_gateway -timeout 10h
2019/10/08 10:44:53 [DEBUG] Running Sweepers for region (us-west-2):
2019/10/08 10:44:53 [DEBUG] Sweeper (aws_vpn_gateway) has dependency (aws_dx_gateway_association), running..
2019/10/08 10:44:53 [DEBUG] Sweeper (aws_dx_gateway_association) has dependency (aws_dx_gateway_association_proposal), running..
2019/10/08 10:44:53 [DEBUG] Running Sweeper (aws_dx_gateway_association_proposal) in region (us-west-2)
2019/10/08 10:44:53 [INFO] Building AWS auth structure
2019/10/08 10:44:53 [INFO] Setting AWS metadata API timeout to 100ms
2019/10/08 10:44:54 [INFO] Ignoring AWS metadata API endpoint at default location as it doesn't return any instance-id
2019/10/08 10:44:54 [INFO] AWS Auth provider used: "SharedCredentialsProvider"
2019/10/08 10:44:54 [DEBUG] Trying to get account information via sts:GetCallerIdentity
2019/10/08 10:44:54 [DEBUG] Trying to get account information via sts:GetCallerIdentity
2019/10/08 10:44:55 [DEBUG] Running Sweeper (aws_dx_gateway_association) in region (us-west-2)
2019/10/08 10:44:55 [INFO] Building AWS auth structure
2019/10/08 10:44:55 [INFO] Setting AWS metadata API timeout to 100ms
2019/10/08 10:44:56 [INFO] Ignoring AWS metadata API endpoint at default location as it doesn't return any instance-id
2019/10/08 10:44:56 [INFO] AWS Auth provider used: "SharedCredentialsProvider"
2019/10/08 10:44:56 [DEBUG] Trying to get account information via sts:GetCallerIdentity
2019/10/08 10:44:56 [DEBUG] Trying to get account information via sts:GetCallerIdentity
2019/10/08 10:44:57 [INFO] Deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association: vgw-0dc65a74a20f0ff68
2019/10/08 10:44:58 [ERROR] Error running Sweeper (aws_dx_gateway_association) in region (us-west-2): error deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association (vgw-0dc65a74a20f0ff68): DirectConnectServerException: Unable to process request
  status code: 400, request id: 12c9dad0-add9-4726-ad39-5d02c529bd4c
FAIL  github.com/terraform-providers/terraform-provider-aws/aws 5.569s
FAIL
$ echo $?
1
```

With new flag:

```console
$ go test ./aws -v -sweep=us-west-2 -sweep-run=aws_vpn_gateway -sweep-allow-failures -timeout 10h
2019/10/08 10:47:13 [DEBUG] Running Sweepers for region (us-west-2):
2019/10/08 10:47:13 [DEBUG] Sweeper (aws_vpn_gateway) has dependency (aws_dx_gateway_association), running..
2019/10/08 10:47:13 [DEBUG] Sweeper (aws_dx_gateway_association) has dependency (aws_dx_gateway_association_proposal), running..
2019/10/08 10:47:13 [DEBUG] Running Sweeper (aws_dx_gateway_association_proposal) in region (us-west-2)
2019/10/08 10:47:15 [DEBUG] Running Sweeper (aws_dx_gateway_association) in region (us-west-2)
2019/10/08 10:47:16 [INFO] Deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association: vgw-0dc65a74a20f0ff68
2019/10/08 10:47:17 [ERROR] Error running Sweeper (aws_dx_gateway_association) in region (us-west-2): error deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association (vgw-0dc65a74a20f0ff68): DirectConnectServerException: Unable to process request
  status code: 400, request id: df997347-77eb-49d4-8a8f-2d3fd6bc7b59
2019/10/08 10:47:17 [ERROR] Error running Sweeper (aws_dx_gateway_association) in region (us-west-2): error deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association (vgw-0dc65a74a20f0ff68): DirectConnectServerException: Unable to process request
  status code: 400, request id: df997347-77eb-49d4-8a8f-2d3fd6bc7b59
2019/10/08 10:47:17 [DEBUG] Running Sweeper (aws_vpn_gateway) in region (us-west-2)
2019/10/08 10:47:18 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-0d83012901b4c0903",
  VpnGatewayId: "vgw-068137a16116b907b"
}
2019/10/08 10:47:18 [DEBUG] Waiting for VPN Gateway (vgw-068137a16116b907b) to detach from VPC (vpc-0d83012901b4c0903)
2019/10/08 10:47:18 [DEBUG] Waiting for state to become: [detached]
2019/10/08 10:52:15 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-068137a16116b907b"
}
2019/10/08 10:52:16 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-0c62fd27265656c2e",
  VpnGatewayId: "vgw-01b70692831191788"
}
2019/10/08 10:52:16 [DEBUG] Waiting for VPN Gateway (vgw-01b70692831191788) to detach from VPC (vpc-0c62fd27265656c2e)
2019/10/08 10:52:16 [DEBUG] Waiting for state to become: [detached]
2019/10/08 10:57:23 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-01b70692831191788"
}
2019/10/08 10:57:24 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-05d5a615959157f27",
  VpnGatewayId: "vgw-0aa8c6fae0a604e10"
}
2019/10/08 10:57:24 [DEBUG] Waiting for VPN Gateway (vgw-0aa8c6fae0a604e10) to detach from VPC (vpc-05d5a615959157f27)
2019/10/08 10:57:24 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:02:32 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-0aa8c6fae0a604e10"
}
2019/10/08 11:02:32 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-0d3407641bd047759",
  VpnGatewayId: "vgw-07576dd2891f26542"
}
2019/10/08 11:02:32 [DEBUG] Waiting for VPN Gateway (vgw-07576dd2891f26542) to detach from VPC (vpc-0d3407641bd047759)
2019/10/08 11:02:32 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:07:50 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-07576dd2891f26542"
}
2019/10/08 11:07:51 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-06287cc0559c30024",
  VpnGatewayId: "vgw-01631d746dfe99ebf"
}
2019/10/08 11:07:51 [DEBUG] Waiting for VPN Gateway (vgw-01631d746dfe99ebf) to detach from VPC (vpc-06287cc0559c30024)
2019/10/08 11:07:51 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:12:59 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-01631d746dfe99ebf"
}
2019/10/08 11:12:59 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-08b202d4b25465bdc",
  VpnGatewayId: "vgw-0ed99c4a2b8d6a339"
}
2019/10/08 11:13:00 [DEBUG] Waiting for VPN Gateway (vgw-0ed99c4a2b8d6a339) to detach from VPC (vpc-08b202d4b25465bdc)
2019/10/08 11:13:00 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:18:07 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-0ed99c4a2b8d6a339"
}
2019/10/08 11:18:07 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-09424d73ee3dfb159",
  VpnGatewayId: "vgw-06169f1efed128de4"
}
2019/10/08 11:18:08 [DEBUG] Waiting for VPN Gateway (vgw-06169f1efed128de4) to detach from VPC (vpc-09424d73ee3dfb159)
2019/10/08 11:18:08 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:23:15 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-06169f1efed128de4"
}
2019/10/08 11:23:16 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-0401f3706243dad85",
  VpnGatewayId: "vgw-0dc65a74a20f0ff68"
}
2019/10/08 11:23:16 [DEBUG] Waiting for VPN Gateway (vgw-0dc65a74a20f0ff68) to detach from VPC (vpc-0401f3706243dad85)
2019/10/08 11:23:16 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:28:23 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-0dc65a74a20f0ff68"
}
2019/10/08 11:28:24 [DEBUG] Detaching VPN Gateway: {
  VpcId: "vpc-09e0bf3c6ea53ee5f",
  VpnGatewayId: "vgw-0f26b1614d922a13d"
}
2019/10/08 11:28:24 [DEBUG] Waiting for VPN Gateway (vgw-0f26b1614d922a13d) to detach from VPC (vpc-09e0bf3c6ea53ee5f)
2019/10/08 11:28:24 [DEBUG] Waiting for state to become: [detached]
2019/10/08 11:33:42 [DEBUG] Deleting VPN Gateway: {
  VpnGatewayId: "vgw-0f26b1614d922a13d"
}
2019/10/08 11:33:42 Sweeper Tests ran successfully:
  - aws_dx_gateway_association_proposal
  - aws_vpn_gateway
2019/10/08 11:33:42 Sweeper Tests ran unsuccessfully:
  - aws_dx_gateway_association: error deleting Direct Connect Gateway (5a00fd97-9872-447d-822c-d2a6245e22d6) Association (vgw-0dc65a74a20f0ff68): DirectConnectServerException: Unable to process request
  status code: 400, request id: df997347-77eb-49d4-8a8f-2d3fd6bc7b59
FAIL  github.com/terraform-providers/terraform-provider-aws/aws 2790.472s
FAIL
$ echo $?
1
```

…global variables, add testing for allowing failures

Reference: #198 (review)