404 Error not translated

application/modules/opac/views/scripts/error/404.phtml

 <div style="text-align:center; font-size:3em">404</div>
 <h1 style="text-align:center">
 <?php 
-echo $this->exception->getMessage();
+echo $this->_('Désolé, cette page n\'existe pas');
 ?>
 </h1>