Allow deletion of backups where the service is None

Allow deletion of backups where the service is None

If a backup is created while both cinder-backup and rabbitmq
are not running, backup records are created in the database with a
service entry of None.

There is no actual backup data created on the service, since the service
isn't running. This fix allows removal of these records without an error
when a delete backup request is received.

This fix also ensures backup status is set to error in the event of
an exception during a delete.

Fixes bug #1162908

Change-Id: I5771747a00a70621f7cc101f8c1da2f613e83cdf