title: @UniqueEntity custom message not translated
body: <p>How can I translate a message in a @UniqueEntity constraint?</p>
  
  <p>I have this situation:</p>
  
  <pre><code>/**
   * @ORM\Entity
   * @ORM\Table(name="sites")
   * @UniqueEntity(
   *     fields={"url"},
   *     message="This url is already registered. Please choose a different url."
   *     )
   */
  class Site
  {
  </code></pre>
  
  <p>If I leave the default message ("This value is already used.") it gets automatically translated with some bundled strings... but I can't find any documentation about how to translate the custom error messages. (I tried to put that string in the default domain, <code>messages</code>, but it doesn't get translated)</p>