* if error is in xmodule_constructor(), catch and return an ErrorModule
* if error is somewhere else in get_module(), return None

Adding a course-specific css class to body so that we can have course-targeted styles as a last resort

* Save errors for courses that failed to load in modulestore
* global staff can see course errors on their dashboard
* put policy loading errors into the error trackers
* add has_access(user, 'global', 'staff'), which is equiv to user.is_staff for now

* with backcompat location /grading_policy.json

* still backcompat with old mode

Allowing filesubmissions to specify which files are allowed and require that certain files are present.

* apply policy per-course, even if multiple courses share course and org fields
* keep descriptors separate in xml store, so that if two such courses
  change the same module in different ways, it works.  Such edits will need to
  merged on CMS import...
* add get_instance(course_id, location) method to replace get_item(location).
  Update all the call sites
* tests, including a 2nd toy course with same course and org.