Terragrunt Panics when Checking an S3 Bucket for Which Versioning is Not Enabled

[josh-padnick]

When you run terragrunt where a .terragrunt file specifies an S3 Bucket that exists but for which versioning has not been enabled, it panics when instead it should issue a warning. Here are the steps to reproduce.

1. Create an S3 Bucket but do NOT enable versioning.
2. Specify that S3 Bucket Name in a .terragrunt file.
3. Run terragrunt plan or terragrunt apply.
4. Terragrunt will panic.

I received this output:

> terragrunt plan
[terragrunt] 2016/12/05 01:30:59 Reading Terragrunt config file at .terragrunt
[terragrunt] 2016/12/05 01:30:59 Initializing remote state for the s3 backend
[terragrunt] 2016/12/05 01:31:00 runtime.errorString runtime error: invalid memory address or nil pointer dereference
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/errors/errors.go:70 (0x69e31)
/usr/local/go/src/runtime/asm_amd64.s:472 (0x59d8e)
	call32: MOVQ	DX, 0(SP);			\
/usr/local/go/src/runtime/panic.go:443 (0x2ba89)
	gopanic: freedefer(d)
/usr/local/go/src/runtime/panic.go:62 (0x2a145)
	panicmem: panic(memoryError)
/usr/local/go/src/runtime/sigpanic_unix.go:24 (0x40a39)
	sigpanic: panicmem()
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/remote/remote_state_s3.go:113 (0xca055)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/remote/remote_state_s3.go:49 (0xc96b1)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/remote/remote_state.go:45 (0xc83c8)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/remote/remote_state.go:60 (0xc85a7)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/cli/cli_app.go:199 (0x68601)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/cli/cli_app.go:102 (0x67475)
/usr/local/go/src/runtime/asm_amd64.s:472 (0x59d8e)
	call32: MOVQ	DX, 0(SP);			\
/usr/local/go/src/reflect/value.go:435 (0x12bc5d)
	Value.call:
/usr/local/go/src/reflect/value.go:303 (0x12a921)
	Value.Call: }
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/vendor/github.com/urfave/cli/app.go:480 (0xd13be)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/vendor/github.com/urfave/cli/app.go:241 (0xceac5)
/home/ubuntu/.go_workspace/src/github.com/gruntwork-io/terragrunt/main.go:19 (0x20ce)
/usr/local/go/src/runtime/proc.go:188 (0x2d5a0)
	main: // Make racy client program work: if panicking on
/usr/local/go/src/runtime/asm_amd64.s:1998 (0x5c441)
	goexit: endofpage:

The fix is straightforward. I'll submit a PR for it now, but would prefer to fix #64 so that we can write a failing test for it.