title: Add error to Symfony 2 form element
body: <p>I check some validation in my controller. And I want to add error to specific element of my form on failure. My form:</p>
  
  <pre><code>use Symfony\Component\Form\FormError;
  
  // ...
  
  $config = new Config();
  $form = $this-&gt;createFormBuilder($config)
          -&gt;add('googleMapKey', 'text', array('label' =&gt; 'Google Map key'))
          -&gt;add('locationRadius', 'text', array('label' =&gt; 'Location radius (km)'))
          -&gt;getForm();
  
  // ...
  
  $form-&gt;addError(new FormError('error message'));
  </code></pre>
  
  <p>addError() method adds error to form, not to element. How can I add an error to locationRadius element?</p>