title: symfony2 and throwing exception error
body: <p>I am trying to throw exceptions and I am doing the following:</p>
  
  <pre><code>use Symfony\Component\HttpKernel\Exception\HttpNotFoundException;
  use Symfony\Component\Security\Core\Exception\AccessDeniedException;
  </code></pre>
  
  <p>I am then using them the following way:</p>
  
  <pre><code> throw new HttpNotFoundException("Page not found");
     throw $this-&gt;createNotFoundException('The product does not exist');
  </code></pre>
  
  <p>however I am getting errors like HttpNotFoundException is not found etc.</p>
  
  <p>Is this the best way to throw exceptions?</p>