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.
All of these RetryErrors were meant to fail right away, but instead
caused retry looping because the typecheck in the implementation of
`resource.Retry()` only catches the value type, and not the pointer
type.
Refs #5537
Paul Hinze