Improve error message when no region provided

[gdavison]

The error message returned when no region parameter was found in any of the sources needs improvement.

For example, the AWS Provider returns:

Error: error configuring Terraform AWS Provider: error validating provider credentials: error calling sts:GetCallerIdentity: operation error STS: GetCallerIdentity, failed to resolve service endpoint, an AWS region is required, but was not found

We may need to either

• validate that we have a region before trying to call sts:GetCallerIdentity, or
• check the error type returned by the AWS SDK