loading state and error handling refactor

lib/view/pages/talk/join_chat.dart

@@ -2,9 +2,11 @@
 import 'package:async/async.dart';
 import 'package:flutter/material.dart';
 
+import '../../../api/errors/error_mapper.dart';
 import '../../../api/marianumcloud/autocomplete/autocompleteApi.dart';
 import '../../../api/marianumcloud/autocomplete/autocompleteResponse.dart';
 import '../../../model/endpoint_data.dart';
+import '../../../widget/app_progress_indicator.dart';
 import '../../../widget/placeholder_view.dart';
 
 class JoinChat extends SearchDelegate<String> {
@@ -16,17 +18,9 @@ class JoinChat extends SearchDelegate<String> {
         future: future!.value,
         builder: (context, snapshot) {
           if(snapshot.connectionState != ConnectionState.done) {
-            return Container(
-              padding: const EdgeInsets.all(10),
-              child: const Center(
-                child: SizedBox(
-                  height: 20,
-                  width: 20,
-                  child: CircularProgressIndicator(
-                    strokeWidth: 3,
-                  ),
-                ),
-              ),
+            return const Padding(
+              padding: EdgeInsets.all(10),
+              child: Center(child: AppProgressIndicator.medium()),
             );
           }
           return const SizedBox.shrink();
@@ -76,10 +70,13 @@ class JoinChat extends SearchDelegate<String> {
             }
           );
         } else if(snapshot.hasError) {
-          return const PlaceholderView(icon: Icons.search_off, text: 'Ein fehler ist aufgetreten. Bist du mit dem Internet verbunden?');
+          return PlaceholderView(
+            icon: Icons.search_off,
+            text: errorToUserMessage(snapshot.error),
+          );
         }
 
-        return const Center(child: CircularProgressIndicator());
+        return const Center(child: AppProgressIndicator.large());
       },
     );
   }