title: Custom exception from Messenger handler
body: <p>I try Symfony 4.3.0-dev version to get some new features from Messenger component. My command bus works in sync mode. </p>
  
  <p>Before upgrading I could easily throw my custom exception <code>ConflictException</code> from handler. But for 4.3.0-dev I get a <code>Symfony\Component\Messenger\Exception\HandlerFailedException</code>.</p>
  
  <p>How can I catch my custom exception again?</p>