If you click on the name of a course which produced an error you should get a list of all the elements of that course showing you how far it got before it died.
To be honest, you shouldn't rely on Moodle's automated backups as your primary backup system, as for some reason it just isn't very reliable. This seems particularly true for larger sites, where there's more chance of the process timing out. You should really be using another method of backing up your Moodle installation, most likely to tape overnight. If you back up your moodledata directory and your MySQL database you should be able to restore your installation in the event of server failure.
Chris