forgot to return dictionary on get_errored_courses

common/lib/xmodule/xmodule/modulestore/mixed.py

@@ -142,3 +142,4 @@ class MixedModuleStore(ModuleStoreBase):
         errs = {}
         for store in self.modulestores.values():
             errs.update(store.get_errored_courses())
+        return errs