Allow retry exception to propagate from ses callback task
Allow retry exception to propagate from ses callback task

Celery `self.retry` raises an exception to communicate that the task
needs to be retried. Since our ses task is wrapped in a catch-all
except block it logs that exception as an error before retrying.

Handling Retry class separately allows us to raise it without logging
the traceback.

alphagov/notifications-api,alphagov/notifications-api