Cakephp如何从模型中获取回调验证错误消息

public function validationDefault(Validator $validator)
{
    $validator
        ->notEmptyString('username', '用户名不能为空')
        ->add('username', 'unique', [
            'rule' => 'validateUnique',
            'provider' => 'table',
            'message' => '用户名已存在',
        ]);

    return $validator;
}

public function add()
{
    $user = $this->Users->newEmptyEntity();
    if ($this->request->is('post')) {
        $user = $this->Users->patchEntity($user, $this->request->getData());
        if ($this->Users->save($user)) {
            $this->Flash->success(__('The user has been saved.'));
            return $this->redirect(['action' => 'index']);
        } else {
            $this->Flash->error(__('The user could not be saved. Please, try again.'));
        }
    }
    $this->set(compact('user'));
}

echo $this->Form->control('username');
echo $this->Form->error('username');