GXRIMO...Instead of monitoring for 1120 events you should be monitoring for 1121 which indicates a backup has failed.
Agent Time: 9:00:00 am 13-Apr-10
Event Time: 2:00:00 pm 13-Apr-10 UTC
Description: Backup status: failed
In addition, I would utilize the ShadowProtect agent in the console to alert you if a backup failed.
... just for completion ...
> Error Codes from ShadowProtect
Thanks Kai. When I asked them about it a few years ago they didn't have any windows logs for image manager so it's great to see that has changed.