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.
This will retry deleting a server cert
if it throws an error about being in use with an ELB (that we've likely just
deleted)
Includes test for ELB+IAM SSL cert bug dependency violation
Clint