title: number input and invalid message in Symfony 2
body: <p>I dont know if I understood it well... if i want an input of my form to receive numbers only, I can use a number field</p>
  
  <pre><code>$builder-&gt;add('width', 'number', array(
          'label' =&gt; $this-&gt;translator-&gt;trans('image.builder.width'),
          'invalid_message' =&gt; 'oops'
      ));
  </code></pre>
  
  <p>When I type letters in the input zone, the error occurs but the message does not display... there is nothing.</p>
  
  <p>Is it the right way to use the number input ?</p>
  
  <p>Thank you</p>