opt: sealed LoadingState (#765)

lib/pages/follow/child_view.dart

@@ -113,7 +113,6 @@ class _FollowChildPageState extends State<FollowChildPage>
           errMsg: loadingState.errMsg,
           onReload: _followController.onReload,
         ),
-      _ => throw UnimplementedError(),
     };
   }
 

lib/pages/follow/view.dart

@@ -147,7 +147,6 @@ class _FollowPageState extends State<FollowPage> {
         ),
       Error() => FollowChildPage(
           controller: _followController, mid: _followController.mid),
-      _ => throw UnimplementedError(),
     };
   }