Add an empty get_errored_courses function for the mongo modulestore

common/lib/xmodule/xmodule/modulestore/mongo.py

@@ -316,3 +316,9 @@ class MongoModuleStore(ModuleStoreBase):
                                     {'_id': True})
         return [i['_id'] for i in items]
 
+    def get_errored_courses(self):
+        """
+        This function doesn't make sense for the mongo modulestore, as courses
+        are loaded on demand, rather than up front
+        """
+        return {}