Validacion en el componente Config

gistfile1.php

/**
 * Configuracion en archivo config.yml
 * 
 * mi_bundle:
 *  config_file: "path/to/file.ini"
 *
 */

class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('mi_bundle');
        $default = "/default/path/to/file.ini";

        $rootNode
            ->children()
                ->scalarNode("config_file")
                    ->defaultValue($default)
                    ->validate()
                        ->ifTrue(function($v){ return !file_exists($v);  })
                        ->thenInvalid("The config file %s doesn't exist")
                    ->end()
                ->end()
            ->end();

        return $treeBuilder;
    }

}