Grade items restored incorrectly cause recursion, memory limit exceeded error

Refer to MDL-37672. Restoring a course backup made in 1.9 which is missing all grade categories, but still has manual grade items that reference those absent categories, will incorrectly insert the grade items with a null category id.

This will cause a recursion/memory exhaustion error when trying to access the course's Grades interface, as the null category id is unanticipated.

Suggested fix is to not allow an import of a manual grade item with a null category id. If the mapping (backup_ids_temp) fails, use the course's grade category as a default.