if( $form->isValid() )
{
// ...
}
else
{
// get a ConstraintViolationList
$errors = $this->get('validator')->validate( $user );
$result = '';
// iterate on it
foreach( $errors as $error )
{
// Do stuff with:
// $error->getPropertyPath() : the field that caused the error
// $error->getMessage() : the error message
}
}