title: Sonata Admin, custom flashBag
body: <p>I have a formMapper Sonata. I have add the prePersist and preUpdate function who are performing some checks before approving changes.</p>
  
  <p>If it fail, I would like to display a nice error message. I have seen that there is some flags who can use but nothing really interesting. Even more, a return false does not stop the action.</p>
  
  <p>Des someone know how could I manage this feature ?</p>
  
  <p>Thanks a lot.</p>