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(self, view, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1914, in render
    return self.__getattr__('render')(block, view_name, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1335, in render
    return super(MetricsMixin, self).render(block, view_name, context=context)
  File "/usr/local/lib/python2.7/site-packages/xblock/runtime.py", line 831, in render
    frag = view_fn(context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/seq_module.py", line 262, in student_view
    return self._student_or_public_view(context, prereq_met, prereq_meta_info, banner_text)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/seq_module.py", line 343, in _student_or_public_view
    items = self._render_student_view_for_items(context, display_items, fragment, view) if prereq_met else []
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/seq_module.py", line 485, in _render_student_view_for_items
    rendered_item = item.render(view, context)
  File "/usr/local/lib/python2.7/site-packages/xblock/core.py", line 202, in render
    return self.runtime.render(self, view, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1914, in render
    return self.__getattr__('render')(block, view_name, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1335, in render
    return super(MetricsMixin, self).render(block, view_name, context=context)
  File "/usr/local/lib/python2.7/site-packages/xblock/runtime.py", line 831, in render
    frag = view_fn(context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/vertical_block.py", line 118, in student_view
    return self._student_or_public_view(context, STUDENT_VIEW)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/vertical_block.py", line 85, in _student_or_public_view
    rendered_child = child.render(view, child_block_context)
  File "/usr/local/lib/python2.7/site-packages/xblock/core.py", line 202, in render
    return self.runtime.render(self, view, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1914, in render
    return self.__getattr__('render')(block, view_name, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1335, in render
    return super(MetricsMixin, self).render(block, view_name, context=context)
  File "/usr/local/lib/python2.7/site-packages/xblock/runtime.py", line 831, in render
    frag = view_fn(context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/library_content_module.py", line 314, in student_view
    rendered_child = displayable.render(STUDENT_VIEW, child_context)
  File "/usr/local/lib/python2.7/site-packages/xblock/core.py", line 202, in render
    return self.runtime.render(self, view, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1914, in render
    return self.__getattr__('render')(block, view_name, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1335, in render
    return super(MetricsMixin, self).render(block, view_name, context=context)
  File "/usr/local/lib/python2.7/site-packages/xblock/runtime.py", line 835, in render
    updated_frag = self.wrap_xblock(block, view_name, frag, context)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/x_module.py", line 1287, in wrap_xblock
    frag = wrapper(block, view, frag, context)
  File "/openedx/edx-platform/openedx/core/lib/xblock_utils/__init__.py", line 155, in wrap_xblock
    add_webpack_to_fragment(frag, class_name)
  File "/openedx/edx-platform/common/lib/xmodule/xmodule/util/xmodule_django.py", line 28, in add_webpack_to_fragment
    for chunk in webpack_loader.utils.get_files(bundle_name, extension, config):
  File "/usr/local/lib/python2.7/site-packages/webpack_loader/utils.py", line 31, in get_files
    return list(_get_bundle(bundle_name, extension, config))
  File "/usr/local/lib/python2.7/site-packages/webpack_loader/utils.py", line 23, in _get_bundle
    bundle = get_loader(config).get_bundle(bundle_name)
  File "/usr/local/lib/python2.7/site-packages/webpack_loader/loader.py", line 83, in get_bundle
    raise WebpackBundleLookupError('Cannot resolve bundle {0}.'.format(bundle_name))
WebpackBundleLookupError: Cannot resolve bundle NonStaffErrorModule.