title: How to set flash message in sonata admin Admin Controller
body: <p>im looking for a way to set flash message in admin controller of sonata admin bundle, they allow to set flash messages in CRUDController as</p>
  
  <pre><code>$this-&gt;get('session')-&gt;setFlash('sonata_flash_error', 'flash_batch_merge_error');
  </code></pre>
  
  <p>but not in the Admin Controller, </p>
  
  <p>this is my admin contrller</p>
  
  <pre><code>use Sonata\AdminBundle\Admin\Admin;
  use Sonata\AdminBundle\Datagrid\ListMapper;
  use Sonata\AdminBundle\Datagrid\DatagridMapper;
  use Sonata\AdminBundle\Form\FormMapper;
  use Sonata\AdminBundle\Route\RouteCollection;
  
  class ConfigAdmin extends Admin
  {
  
  protected function configureFormFields(FormMapper $formMapper)
  {   
  
      $formMapper
          -&gt;with('System Settings')
              -&gt;add('Name','text', array('label' =&gt; "Configuration Name"))
              -&gt;add('Language', 'choice', array(
                  'label' =&gt; 'System Language',
                  'choices' =&gt; array(0 =&gt; 'English', 1 =&gt; 'Swedish'),
                  'preferred_choices' =&gt; array(0),
                  ))
              -&gt;add('commonmail','text', array('label' =&gt; "Common e-Mail"))
              -&gt;add('dateformat','text', array('label' =&gt; "Date format"))
              -&gt;add('currencyformat','text', array('label' =&gt; "Currency format"))
          -&gt;end()
  }
  
  public function postUpdate($object) {
  
        // here i need to perform some validations and set flash message if there is an errror 
  
  }
  
  }
  </code></pre>
  
  <p>appreciate your help</p>