This task didn't have retries before, based on the assumption that if
the task failed it was likely to be due to a Boto error, so retrying
would not help because a file was probably not in the expected bucket.
During an incident with the database, we had some letters that were
stuck in the `pending-virus-check` state because this task failed.
This change adds retries to the task if there was an Exception that was
not a `BotoClientError`.