If an error occurs while rendering an xmodule, the LMS chokes with the
following error if accessed by a non admin user.

In the admin user case the existing line that registers ErrorDescriptor
ensures that everything works. But for the non admin case we need this
additional directive.

Traceback (most recent call last):
  File "/openedx/edx-platform/lms/djangoapps/courseware/views/index.py", line 153, in get
    return self.render(request)
  File "/openedx/edx-platform/lms/djangoapps/courseware/views/index.py", line 217, in render
    return render_to_response('courseware/courseware.html', self._create_courseware_context(request))
  File "/openedx/edx-platform/lms/djangoapps/courseware/views/index.py", line 468, in _create_courseware_context
    courseware_context['fragment'] = self.section.render(self.view, section_context)
  File "/usr/local/lib/python2.7/site-packages/xblock/core.py", line 202, in render
    return self.runtime.render...