feat(login error handling): Added invalid crendential error

Author: DK10WS

frontend/lib/pages/login.dart

@@ -113,6 +113,9 @@ class _MyLoginState extends State<MyLogin> {
         SessionManager.setSession(cookies, asp);
 
         return {'name': name ?? '', 'newCookies': cookies};
+      }
+      if (response.statusCode == 401) {
+        _showError(response.data["detail"]);
       } else {
         _showError("Login failed: ${response.data["message"]}");
         return null;

frontend/lib/utils/util.dart

@@ -29,7 +29,7 @@ Future<Response> post(String url, Map<String, String> postheaders,
       options: Options(
         headers: postheaders,
         contentType: Headers.jsonContentType,
-        validateStatus: (status) => status! < 400,
+        validateStatus: (status) => status! < 500,
       ));
   return response;
 }