provider/aws: Check credentials before attempting to do anything

[catsby]

Attempt to get the current user through IAM, in effort to verify the credentials work at all prior to doing anything.

Before:

$ AWS_SECRET_ACCESS_KEY=nope terraform plan
[...]

+ aws_instance.tf_test
    ami:                         "" => "ami-6ff2f15f"
  [....] # a bunch of planned things

$ AWS_SECRET_ACCESS_KEY=nope terraform apply
aws_key_pair.ssh_thing: Creating...
  [...]
aws_key_pair.ssh_thing: Error: 1 error(s) occurred:

* Error import KeyPair: AuthFailure: AWS was not able to validate the provided access credentials
    status code: 401, request id: []
aws_vpc.default: Error: 1 error(s) occurred:

* Error creating VPC: AuthFailure: AWS was not able to validate the provided access credentials
    status code: 401, request id: []
Error applying plan:

3 error(s) occurred:

* Error import KeyPair: AuthFailure: AWS was not able to validate the provided access credentials
    status code: 401, request id: []
* Error creating VPC: AuthFailure: AWS was not able to validate the provided access credentials
    status code: 401, request id: []
* Resource 'aws_internet_gateway.gw' does not have attribute 'id' for variable 'aws_internet_gateway.gw.id'

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.

After:

$ AWS_SECRET_ACCESS_KEY=nope terraform plan
Refreshing Terraform state prior to plan...

Error refreshing state: 1 error(s) occurred:

* 1 error(s) occurred:

* Failed authenticating with AWS: please verify credentials

Note that the bad credentials are caught at the plan stage.

Fixes #2703

[phinze]

Ooo this is a great idea. 👍

[radeksimko]

👍

[catsby]

heh, idea credit to @gtmtech 😄

[ghost]

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