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.
```
go tool vet -all .
builtin/providers/aws/resource_aws_elasticache_security_group.go:130: arg apierr.Code in printf call is a function value, not a function call
builtin/providers/aws/resource_aws_elasticache_subnet_group.go:155: arg apierr.Code in printf call is a function value, not a function call
```
This landed in aws-sdk-go yesterday, breaking the AWS provider in many places:
3c259c9586
Here, with much sedding, grepping, and manual massaging, we attempt to
catch Terraform up to the new `awserr.Error` interface world.
Joshua Spence