ui: improve error

ui/lib/activity.dart

@@ -148,7 +148,7 @@ class _ActivityPageState extends ConsumerState<ActivityPage>
                   },
                 ));
               },
-              error: (err, trace) => PoError(msg: "网络错误", err: err),
+              error: (err, trace) => PoNetworkError(err: err),
               loading: () => const MyProgressIndicator());
         })
       ],