title: UniqueEntity on multiple fields doesn't show an error message
body: <p>I've added the following constraint to my entity:</p>
  
  <pre><code> * @ORM\Table(name="link", uniqueConstraints={@ORM\UniqueConstraint(name="unique_link_idx", columns={"first_form_id", "first_question_id","last_form_id","last_question_id"})})
   * @UniqueEntity(fields={"firstForm", "firstQuestion","lastForm","lastQuestion"}, message="Cette liaison existe déjà !")
   * @ORM\Entity(repositoryClass="Ineat\LinkQuestionBundle\Entity\LinkRepository")
  </code></pre>
  
  <p>The UniqueEntity constraint works well because before adding it, when trying to insert the exact same entity to the DB I had a DBALException abount integrity constraint.
  Since then I havn't this exception anymore, the form isn't validating but <code>{{ form_errors(form) }}</code> doesn't print any message.</p>
  
  <p>After having checked in the controller <code>$form-&gt;getErrors()</code> returns an empty array while <code>$form-&gt;getErrorsAsString()</code> contains the UniqueENtity constraint.</p>
  
  <p>How can I have my UniqueEntity's constraint error message showing</p>