component AP gateway: be sure to have a string for error message.

libervia/backend/plugins/plugin_comp_ap_gateway/pubsub_service.py

@@ -430,7 +430,7 @@ class APPubsubService(rsm.PubSubService):
                 except Exception as e:
                     raise error.StanzaError(
                         "item-not-found",
-                        text=e
+                        text=str(e)
                     )
             else:
                 actor_data = await self.apg.get_ap_actor_data_from_account(ap_account)