Twig中的{{exception.message}}不呈现HTML

I have this controller where \Exception is raised (I haven't figured out which SF2 Exception to use yet) upon certain condition. Here is it:

<?php

namespace My\AppBundle\Controller;

use ....

class MyController extends Controller
{
     const EXCEPTION_MESSAGE = <<<EOF
My <b>HTML</b>
<br/>
<small>Small phrase</small>
EOF;

     public function indexAction()
     {
         // my logic

         if(in_array($data, $array))
             throw new \Exception(self::EXCEPTION_MESSAGE);

         // the rest of my logic

         return ....
     }

}

And in app/Resources/TwigBundle/views/Exception/error.html.twig

{% extends '::base.html.twig' %}

{% block body %}
    <h2>Error</h2>
    <p>{{ exception.message }}</p>
{% endblock %}

The problem is HTML is not rendered when seeing the error page in prod environement.

I tried {{ exception.message|raw }} and also setting autoescape to false as per this answer but it seems to have no effect.

How can I do to make HTML works when displaying the \Exception message in Twig?

Where ever in the code you catch the exception is where it is needed to be added to the array you pass to twig. for example

$vars = [];
try {

    $a->indexAction();
    //fill $vars 


} catch (Exception $e) {
  $vars['exception'] = $e;
}

//pass $vars to twig