Check service user credentials outside of task

[phwissmann]

          you're right in that it's an inconsistent state. I didn't really want the transfer to be impeded by it because it can "technically" be corrected post transfer, but it might be better to abandon the request as is. But in that case, maybe this check should be in the initial request for ingestion, not part of the transfer task.

Originally posted by @consolethinks in #36 (comment)