title: How to apply a class to a validation error message?
body: <p>I have a Simple Form, as it's shown in the <strong>UserType.php</strong> code source. </p>
  
  <p>Besides, i'm using <strong>fosuserbundle</strong>, in my case, when i try to create a user that already exist with same username or a user with different password/Repeatedpassword , a new validation message is shown up .</p>
  
  <p><strong>The question is :</strong> How to apply custom CSS/Html Class to that validation message ?.<br><br></p>
  
  <blockquote>
    <p>UserType.php :</p>
  </blockquote>
  
  <pre><code>-&gt;add('plainPassword', RepeatedType::class, array(
  
      'invalid_message' =&gt; 'The passwords must be identical.',
  
      'first_options'  =&gt; array('label' =&gt; 'Password'),
      'second_options' =&gt; array('label' =&gt; 'Repeate Password'),
  )
  </code></pre>
  
  <blockquote>
    <p>User.php :</p>
  </blockquote>
  
  <pre><code>&lt;?php
  
  namespace AppBundle\Entity;
  
  use FOS\UserBundle\Model\User as BaseUser;
  use Doctrine\ORM\Mapping as ORM;
  use Doctrine\Common\Collections\ArrayCollection;
  use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
  use Symfony\Component\HttpFoundation\File\File;
  use Vich\UploaderBundle\Mapping\Annotation as Vich;
  
  /**
   * @Vich\Uploadable
   * @ORM\Entity
   * @ORM\Table(name="fos_user")
   * @UniqueEntity(fields="usernameCanonical", errorPath="username", message="fos_user.username.already_used", groups={"Default", "Registration", "Profile"})
   * @UniqueEntity(fields="emailCanonical", errorPath="email", message="fos_user.email.already_used", groups={"Default", "Registration", "Profile"})
   */
  class User extends BaseUser {
  
      /**
       * @ORM\Id
       * @ORM\Column(type="integer")
       * @ORM\GeneratedValue(strategy="AUTO")
       * 
       */
      protected $id;
  </code></pre>