Fix nova-compute use of missing DBError

Apparently exception.DBError disappeared at some point, but nova-
compute was still using it. Since it was a stretch in the first
place, and nothing else in exception.py looks reasonable, this
adds exception.DBNotAllowed.

Fixes bug 1120390

Change-Id: Ifbfa0b09f3dc011ad87f6f3b06eb0fdaa7247ec0