fix_course_sortorder(): fix breakage with large categories, saner error

fix_course_sortorder(): fix breakage with large categories, saner error

Two improvements for fix_course_sortorder()

 - If the category has more courses than the shift range
   use a larger shift range to avoid overlapping with itself

 - If things do go wrong during the per-course sortorder updates,
   rollback and try and call ourselves with a 'safe' flag.

Still - far from perfect. Probably the global sortorder approach
is broken. The sanest way is to rework things to always join against
course_categories and order by the combined sortorders.