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

* error modules now hidden via access control
* get_module() returns None if user doesn't have access

* cleaned up error module:
  - only one template
  - save error message in xml and reload
* better display of problem definition and metadata to staff
* save error messages as string, not exception objects.

Display specific error messages when parts of the courseware break, rather than returning a 500 indicating that the whole site is down