silverstripe-cms/code/controllers/ErrorPageControllerExtension.php

<?php

/**
 * Enhances error handling for a controller with ErrorPage generated output
 *
 * @package cms
 * @subpackage controller
 */
class ErrorPageControllerExtension extends Extension {

	public function onBeforeHTTPError($statusCode, $request) {
		$response = ErrorPage::response_for($statusCode);
		if($response) {
			throw new SS_HTTPResponse_Exception($response, $statusCode);
		}
	}
}