Add error type check when retrying

[itsjaspermilan]

🛠️ Description

There is an issue where URL Errors would get swallowed by the retry logic put in place for HTTP requests. This makes it hard to debug when a user may have an invalid login configuration so a check was put into place that identifies the type of error that was received and directs the user to a more helpful error message if necessary.

Fixes: #375

🚢 Release Note

* all: Prevents the app from crashing when a `*url.Error` is received while retrying HTTP requests. [GH-376]

🏗️ Acceptance tests

• Are there any feature flags that are required to use this functionality?
• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccAwsHvnOnly'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/... -v -run=TestAccAwsHvnOnly -timeout 210m
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/clients    (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/consul     (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/input      (cached) [no tests to run]
=== RUN   TestAccAwsHvnOnly
--- PASS: TestAccAwsHvnOnly (123.99s)
PASS
ok      github.com/hashicorp/terraform-provider-hcp/internal/provider   124.314s
...

[bcmdarroch]

🎉 Nice quick fix! Looking forward to getting more robust error helpers in the Go SDK to help even more. :)