DS-1733: Improvements to changes made in PR#364. 

* Ensure withdrawn items are still exported to CSV, since this is a feature of Bulk Metadata Editing.
* Improve error logging on export.

Relates to https://jira.duraspace.org/browse/DS-1733 and #364 (merged).

I've done some light testing this afternoon. Did some metadata exports of both in_archive and withdrawn content (in_archive=false). No errors appear in the logs.

I think this is ready to merge