[Database result backend] Fixes JSON serialization of exceptions (Issue #2441)

celery/backends/database/__init__.py

@@ -140,7 +140,7 @@ class DatabaseBackend(BaseBackend):
                 task = Task(task_id)
                 task.status = states.PENDING
                 task.result = None
-            return task.to_dict()
+            return self.meta_from_decoded(task.to_dict())
 
     @retry
     def _save_group(self, group_id, result):