[16][IMP] attachment_queue automatic retry

[florian-dacosta]

For now, when an attachment queue is processsed by a queue.job, there is no automatic retry in case of concurrent update because all errors are caught. And, for the same reason, the RetryableJobError does not work. This PR aims to restore a consistent behavior about this. The only bad point is that, in case of RetryableJobError, the failure email will be sent anyway, even if the job will retry the attachment. It would then be kind of a false positive. I don't think to have any other choice, because at the time we send the mail, we can't really know if the job will retry or not (we do not know if the max_retries will be reached).

@kevinkhao @bealdav @SirAionTech