在Symfony中显示约束验证器错误消息

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints as Assert;

class RegistrationFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('username', null, [
                'constraints' => [
                    new Assert\NotBlank(),
                    new Assert\Unique(['message' => '该用户名已被使用。']),
                ],
            ])
            // 添加其他字段...
        ;
    }
}

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Routing\Annotation\Route;

class RegistrationController extends AbstractController
{
    /**
     * @Route("/register", name="register")
     */
    public function register(Request $request)
    {
        $form = $this->createForm(RegistrationFormType::class);

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // 处理表单提交...
        }

        return $this->render('registration/register.html.twig', [
            'form' => $form->createView(),
        ]);
    }
}

{{ form_start(form) }}
    {{ form_row(form.username) }}
    {{ form_errors(form.username) }}
    {{ form_row(form.otherField) }}
    {{ form_row(form.submit) }}
{{ form_end(form) }}