Handle Heroku's 503 maintenance mode response

The LMS comment client previously would try to parse the response
as JSON, choke, and return a 500 to the client. Now, the LMS client
displays a message indicating that the forums are down for
maintenance.

lms/djangoapps/django_comment_client/forum/views.py

@@ -174,6 +174,9 @@ def forum_form_discussion(request, course_id):
     try:
         unsafethreads, query_params = get_threads(request, course_id)   # This might process a search query
         threads = [utils.safe_content(thread) for thread in unsafethreads]
+    except (cc.utils.CommentClientMaintenanceError) as err:
+        log.warning("Forum is in maintenance mode")
+        return render_to_response('discussion/maintenance.html', {})
     except (cc.utils.CommentClientError, cc.utils.CommentClientUnknownError) as err:
         log.error("Error loading forum discussion threads: %s" % str(err))
         raise Http404

lms/lib/comment_client/utils.py

@@ -55,6 +55,9 @@ def perform_request(method, url, data_or_params=None, *args, **kwargs):
 
     if 200 < response.status_code < 500:
         raise CommentClientError(response.text)
+    # Heroku returns a 503 when an application is in maintenance mode
+    elif response.status_code == 503:
+        raise CommentClientMaintenanceError(response.text)
     elif response.status_code == 500:
         raise CommentClientUnknownError(response.text)
     else:
@@ -72,5 +75,9 @@ class CommentClientError(Exception):
         return repr(self.message)
 
 
+class CommentClientMaintenanceError(CommentClientError):
+    pass
+
+
 class CommentClientUnknownError(CommentClientError):
     pass

lms/templates/discussion/maintenance.html

+<%inherit file="../main.html" />
+<h1>We're sorry</h1>
+<p>The forums are currently undergoing maintenance. We'll have them back up shortly!</p>