title: Check if template exists before rendering
body: <p>is there a way to check if twig template exists before calling to render? A try catch block seems not to work, at least in dev environment, and plus, I prefer a check than the cost of an exception.</p>
  
  <p>This class <a href="http://api.symfony.com/2.0/Symfony/Bundle/TwigBundle/TwigEngine.html">TwigEngine</a> has an exists() method, but didn't find examples on use.</p>