Fix deleting media with attachments.

Fix deleting media with attachments.

If one deletes a media with attachments, there have been
various problems:
1) If the file in the storage did not exist any more (maybe
   because due to a previous deletion attempt?), the error
   propagation failed, because the wrong thing was
   gathered.
2) The attachment database entries were not deleted.
   Using cascade for this, for now.

Also add a simple unit test, that tests both by having a
broken attachment on a media.