silverstripe-cms/code/Controllers/ErrorPageControllerExtension.php

<?php

namespace SilverStripe\CMS\Controllers;

use SilverStripe\CMS\Model\ErrorPage;
use SilverStripe\Control\Director;
use SilverStripe\Control\HTTPRequest;
use SilverStripe\Control\HTTPResponse_Exception;
use SilverStripe\Core\Extension;

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

	/**
	 * Used by {@see RequestHandler::httpError}
	 *
	 * @param int $statusCode
	 * @param HTTPRequest $request
	 * @throws HTTPResponse_Exception
	 */
	public function onBeforeHTTPError($statusCode, $request) {
		if (Director::is_ajax()) {
			return;
		}
		$response = ErrorPage::response_for($statusCode);
		if($response) {
			throw new HTTPResponse_Exception($response, $statusCode);
		}
	}
}