better loading indicators for timetables, talk and files

lib/api/marianumcloud/talk/chat/getChatCache.dart

@@ -8,7 +8,15 @@ import 'getChatResponse.dart';
 class GetChatCache extends RequestCache<GetChatResponse> {
   String chatToken;
 
-  GetChatCache({required void Function(GetChatResponse) onUpdate, required this.chatToken}) : super(RequestCache.cacheNothing, onUpdate) {
+  GetChatCache({
+    required void Function(GetChatResponse) onUpdate,
+    void Function(Exception)? onError,
+    required this.chatToken,
+  }) : super(
+          RequestCache.cacheNothing,
+          onUpdate,
+          onError: onError ?? RequestCache.ignore,
+        ) {
     start('nc-chat-$chatToken');
   }