title: Symfony Form form_rest block not recognized
body: <p>I am using FosUserBundle for an application.
  I am overriding the registration template inside (<code>app/Resources/FOSUserbundle/views/Registration/register.html.twig</code>).</p>
  
  <blockquote>
    <p>register.html.twig</p>
  </blockquote>
  
  <pre><code>{% extends 'base.html.twig' %}
  {% form_theme form '@FOSUser/Form/register_theme.html.twig' %}
  
  {% block body %}
  ...
  {{ form_start(form, {'attr': {'novalidate': 'novalidate'}}) }}
  ...
  {{ form_rest(form) }} 
  //this is just for testing since form_end contains form_rest 
  //inside of it if you have a look at form_div_layout.html.twig.
  
  {{ form_end(form) }}
  ...
  {% endblock body %}
  </code></pre>
  
  <p>and contains a form_theme file where I changed 2 blocks (which are not related to this question)</p>
  
  <blockquote>
    <p>register_theme.html.twig</p>
  </blockquote>
  
  <pre><code>{% extends "form_div_layout.html.twig" %}
  
  {% block form_widget_simple %}
  //some code here
  {% endblock form_widget_simple %}
  </code></pre>
  
  <p>The problem is that all worked fine until 2.8 update (I still don't think it has to do with this but maybe I don't see something).</p>
  
  <p>So when I try to use the form_rest i get the following error:</p>
  
  <pre><code>An exception has been thrown during the rendering of a template ("Unable to render the form as none of the following blocks exist: "_fos_user_registration_form_rest", "fos_user_registration_rest", "fos_user_registration_rest", "form_rest".") in FOSUserBundle:Registration:register.html.twig at line xx
  </code></pre>
  
  <p>If i do not use form_rest the csrf token is missing and i cannot complete the registration.
  If I use only form_end will still generate the error because form_rest is contained inside of it.</p>