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.

mediagoblin/db/models.py

@@ -145,6 +145,7 @@ class MediaEntry(Base, MediaEntryMixin):
         )
 
     attachment_files_helper = relationship("MediaAttachmentFile",
+        cascade="all, delete-orphan",
         order_by="MediaAttachmentFile.created"
         )
     attachment_files = association_proxy("attachment_files_helper", "dict_view",