Feedback message system to store messages on users session and show them on templates. Flash messages and persistent ones, two implementations. It works. Maybe this Gist would rise some errors because of the changes the code needed to make it public. Hope no security holes. Enjoy!

Acme\Bundle\Controller\DefaultController.php

<?php

/*
 * This file is part of the FOSUserBundle package.
 *
 * (c) FriendsOfSymfony <http://friendsofsymfony.github.com/>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Acme\Bundle\FeedbackBundle\Controller;

use Acme\Bundle\FeedbackBundle\Form\Type\FormType as FormType;
use Acme\Bundle\FeedbackBundle\Model\FormModel as FormModel;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\DependencyInjection\ContainerAware;
use Symfony\Component\HttpFoundation\RedirectResponse;

class DefaultController extends ContainerAware
{
    public function indexAction(Request $request)
    {
        $form = $this->createForm(new FormType(),new FormModel())

        if('POST' === $request->getMethod()
        {
            $form->bind($request);

            if ($form->isValid()) {
              $url = $this->container->get('router')->generate('your_redirect_url');
              $response = new RedirectResponse($url);
            }

            // If errors
            $this->noticeFormErrors($form);
            return $this->container->get('templating')->renderResponse('AcmeFeedbackBundle:Demo:form_error.json.twig',array(),new Response('',500));
        }
    }

    private function noticeFormErrors($form)
    {
        foreach($form->getErrors() as $error)
        {
            $opts = array(
                'level' => FlashFeedbackMessage::LEVEL_ERROR,
                'scope' => $form->getName(),
                'text' => $error->getMessage(),
                'html' => $error->getMessage()
            );

            $message = new FlashFeedbackMessage($opts);
            $message->notify($this->get('session'));
        }

        foreach($this->form->getChildren() as $child)
        {
            foreach($child->getErrors() as $error)
            {
                $opts = array(
                    'level' => FlashFeedbackMessage::LEVEL_ERROR,
                    'scope' => $form->getName() . '_' . $child->getName(),
                    'text' => $error->getMessage(),
                    'html' => $error->getMessage()
                );

                $message = new FlashFeedbackMessage($opts);
                $message->notify($this->get('session'));
            }
        }
    }

}

Acme\Bundle\FeedbackBundle\Util\FeedbackMessageInterface.php

<?php

namespace Acme\Bundle\FeedbackBundle\Util\FeedbackMessageInterface.php

use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\OptionsResolver\Options;

abstract class FeedbackMessageInterface
{
    protected $level;
    protected $scope;
    protected $text;
    protected $html;

    const LEVEL_INFO = 'info';
    const LEVEL_SUCCESS = 'success';
    const LEVEL_WARNING = 'warning';
    const LEVEL_ERROR = 'error';

    public function __construct(array $options = array())
    {
        $resolver = new OptionsResolver();
        $this->setDefaultOptions($resolver);
        $this->setRequiredOptions($resolver);

        $this->options = $resolver->resolve($options);

        $this->level = $this->options['level'];
        $this->scope = $this->options['scope'];
        $this->text = $this->options['text'];
        $this->html = $this->options['html'];
    }

    protected function setRequiredOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setRequired(array(
            'scope',
            'text'
        ));
    }

    protected function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'level' => self::LEVEL_INFO,
            'html' => $this->options['text']
        ));
    }
}

Acme\Bundle\FeedbackBundle\Util\FlashFeedbackMessage.php

<?php

namespace Acme\Bundle\FeedbackBundle\Util\Feedback;

use Symfony\Component\HttpFoundation\Session\Session;

class FlashFeedbackMessage extends FeedbackMessageInterface
{
    public $level;
    public $scope;
    public $text;
    public $html;

    public function notify(Session $session)
    {
        $session->getFlashBag()->add('feedback_messages',(array)$this);
    }
}

Acme\Bundle\FeedbackBundle\Util\PersistentFeedbackMessage.php

<?php

namespace Acme\Bundle\FeedbackBundle\Util\Feedback;

use Symfony\Component\HttpFoundation\Session\Session;

class PersistentFeedbackMessage extends FeedbackMessageInterface
{
    protected $id;
    public $level;
    public $scope;
    public $text;
    public $html;

    const ID_EMAIL_NOT_VERIFIED = 'email_not_verified';
    const ID_PROFILE_NOT_COMPLETED = 'profile_not_completed';

    public function __construct(array $options = array())
    {
        parent::__construct($options);

        $this->id = $this->options['id'];
    }

    public function notify(Session $session)
    {
        $session->set('feedback_messages/' . $this->id,(array)$this);
    }

    protected function setRequiredOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setRequired(array(
            'id',
            'scope',
            'text'
        ));

        $resolver->setAllowedValues(array(
            'id' => array(self::ID_PROFILE_NOT_COMPLETED, self::ID_EMAIL_NOT_VERIFIED)
        ));
    }
}

Acme\FeedbackBundle\Resources\views\Demo\form_error.json.twig

{
    messages: [
    {% block messages %}
    {% for message in app.session.flashbag.get('feedback_messages') %}
    {
        level: '{{ message.level }}',
        persist: false,
        scope: '{{ message.scope }}',
        text: '{{ message.text }}',
        html: '{{ message.html }}'
    },
    {% endfor %}

    {% for message in app.session.get('feedback_messages') %}
    {
        level: '{{ message.level }}',
        persist: true,
        scope: '{{ message.scope }}',
        text: '{{ message.text }}',
        html: '{{ message.html }}'
    },
    {% endfor %}

    {% endblock %}
}