Exception not thrown when twig template does not exist

In the new stack, eZ Publish does not properly throw an exception if a template does not exist.

Steps to reproduce:

1. In a controller, try to render a non-existent template:

return $this->render( 'HootsuiteBaseBundle::nonexistent.html.twig' );

Instead of throwing an exception, the output just shows the path to the non-existent template.

2. In a controller, check whether a non-existent template exists:

if( $this->get( 'templating' )->exists( 'eztTrainingBundle::nonexistent.html.twig' ) )
{
	die( 'exists' );
}

The "exists" check always returns true.