fix(mob): explain why editing a message failed

mobile/lib/controllers/chat/active_chat_controller.dart

@@ -435,6 +435,7 @@ class ActiveChatController extends ChangeNotifier {
       );
       return null;
     } catch (e) {
+      debugPrint("Edit message failed: $e");
       return ApiService.errorMessage(e, fallback: "Couldn't edit the message.");
     }
   }

mobile/lib/services/api_services.dart

@@ -64,6 +64,11 @@ class ApiService {
         final message = data['error'] as String;
         return message[0].toUpperCase() + message.substring(1);
       }
+      // A bare 404/405 (not our JSON envelope) means the route doesn't exist,
+      // i.e. the server is older than the app.
+      if ((status == 404 || status == 405) && data is! Map) {
+        return 'This server doesn\'t support that yet. It may need updating.';
+      }
       switch (error.type) {
         case DioExceptionType.connectionTimeout:
         case DioExceptionType.receiveTimeout:

mobile/test/api_error_message_test.dart

@@ -0,0 +1,39 @@
+import 'package:dio/dio.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mobile/services/api_services.dart';
+
+DioException _error(int status, dynamic data) {
+  final options = RequestOptions(path: '/messages/1');
+  return DioException(
+    requestOptions: options,
+    type: DioExceptionType.badResponse,
+    response: Response(requestOptions: options, statusCode: status, data: data),
+  );
+}
+
+void main() {
+  group('ApiService.errorMessage', () {
+    test('prefers the server error field', () {
+      final message = ApiService.errorMessage(
+        _error(404, {'success': false, 'error': 'message not found'}),
+      );
+      expect(message, 'Message not found');
+    });
+
+    test('explains a missing route on an older server', () {
+      final message = ApiService.errorMessage(
+        _error(404, '404 page not found'),
+        fallback: "Couldn't edit the message.",
+      );
+      expect(message, contains('may need updating'));
+    });
+
+    test('falls back for other failures', () {
+      final message = ApiService.errorMessage(
+        _error(500, 'boom'),
+        fallback: "Couldn't edit the message.",
+      );
+      expect(message, "Couldn't edit the message.");
+    });
+  });
+}