Allows users from govcloud and other regions (aws-cn) to now use the following resources correctly:
```
- data "aws_billing_service_account"
- data "aws_elb_service_account"
- resource "aws_cloudfront_origin_access_identity"
- resource "aws_ecs_service"
- resource "aws_iam_saml_provider"
- resource "aws_lambda_permission"
- resource "aws_sns_topic_policy"
```
Change the `RetryFunc` from a plain `error` return type to a
specialized `RetryError` which must decide whether it is
retryable or not.
Add `RetryableError` / `NonRetryableError` factory functions that
callers are meant to use to build up these errors.
This makes it eminently clear whether or not a given error is
retryable from inside the client code.
Goal here is to _not_ change any behavior, simply reflect the
existing behavior with the new, clearer, API.
Brandon Clodius