Catching errors on deletion.

Fixed the fact that error wasn't handled when deleting an item. 
In a project, I had to sync backend with an external Cloud. I used mongoose middleware (pre-remove) to try deleting the same item on the cloud. If something went wrong or deleting is locked (this happens when something is pending), I pass an error to next(). It cancels the deletion, but no message was prompt (flash or console).
Merge pull request #332 from fbeaudet/patch-1

Catching errors on deletion.