Respond with helpful and spec complient error on invalid user credentials

Author: marc-mabe

src/Exception/OAuthServerException.php

@@ -189,7 +189,7 @@ public static function invalidScope($scope, $redirectUri = null)
      */
     public static function invalidCredentials()
     {
-        return new static('The user credentials were incorrect.', 6, 'invalid_credentials', 401);
+        return new static('The user credentials were incorrect.', 6, 'invalid_grant', 400);
     }
 
     /**

src/Grant/PasswordGrant.php

@@ -106,7 +106,7 @@ protected function validateUser(ServerRequestInterface $request, ClientEntityInt
         if ($user instanceof UserEntityInterface === false) {
             $this->getEmitter()->emit(new RequestEvent(RequestEvent::USER_AUTHENTICATION_FAILED, $request));
 
-            throw OAuthServerException::invalidGrant();
+            throw OAuthServerException::invalidCredentials();
         }
 
         return $user;