We got a situation when some custom code broke backup files and particular capabilities in /course/roles.xml

Looking at the code it seems like there is TODO saying "it seems that assign_capability() doesn't check for valid capabilities at all ???"  

If we look at the assign_capability method, it currently checks for that, and throws and exception.

This breaks restore process. I think, it's better to log a warning instead and let the backup process to complete. 

Original Replication instructions:

1. Create a course.
2. Go to Course administration > Users > Permissions
3. Pick one of the roles in Advanced role override
4. Override mod/lesson:addinstance permission
5. Backup the course and download the file.
6. Uninstall mod_lesson and delete mod/lesson folder completely from the code base to make sure that the activity won't be reinstalled.
7. Restore as a new course from the file you got on step 5
8. Notice you get "Coding error detected, it must be fixed by a programmer: Capability 'mod/lesson:addinstance' was not found! This has to be fixed in code."