Make CMS user view not show ErrorDescriptor courses

[ichuang]

If a bad course gets into the mongodb, then it crashes the CMS user view showing the list of courses.

This PR fixes this by suppressing courses which are ErrorDescriptor instances.

Someday in the future, it may be desirable to make ErrorDescriptor instances show up with red flags to staff.

[chrisndodge]

Curious, can you forward what the stack trace on the original exception was?

[ichuang]

Traceback:
File "/home/mitx/mitx_all/python/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/mitx/mitx_all/edx-platform/cms/djangoapps/contentstore/views/public.py" in howitworks
  49.         return index(request)
File "/home/mitx/mitx_all/python/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  20.                 return view_func(request, *args, **kwargs)
File "/home/mitx/mitx_all/python/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/mitx/mitx_all/edx-platform/cms/djangoapps/contentstore/views/user.py" in index
  67.         'courses': [format_course_for_view(c) for c in courses ],
File "/home/mitx/mitx_all/edx-platform/cms/djangoapps/contentstore/views/user.py" in format_course_for_view
  60.             course.display_org_with_default,

Exception Type: AttributeError at /
Exception Value: 'ErrorDescriptor' object has no attribute 'display_org_with_default'