title: Symfony2 customize form error messages
body: <p>can you please help me understand how to customize error messages in a form with Symfony2?
  I want to change the HTML layout, adding div, class, etc...</p>
  
  <p>Reading the guide, it gives a piece of code to put in a file called <code>fields_errors.html.twig</code> but it doesn't tells where to put this file and if some extra configuration is needed.</p>
  
  <p>Can somebody help me?</p>