Http4xxController.php

<?php

namespace Drupal\system\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller for default HTTP 4xx responses.
 */
class Http4xxController extends ControllerBase {
    
    /**
     * The default 4xx error content.
     *
     * @return array
     *   A render array containing the message to display for 4xx errors.
     */
    public function on4xx() {
        return [
            '#markup' => $this->t('A client error happened'),
        ];
    }
    
    /**
     * The default 401 content.
     *
     * @return array
     *   A render array containing the message to display for 401 pages.
     */
    public function on401() {
        return [
            '#markup' => $this->t('Log in to access this page.'),
        ];
    }
    
    /**
     * The default 403 content.
     *
     * @return array
     *   A render array containing the message to display for 403 pages.
     */
    public function on403() {
        return [
            '#markup' => $this->t('You are not authorized to access this page.'),
        ];
    }
    
    /**
     * The default 404 content.
     *
     * @return array
     *   A render array containing the message to display for 404 pages.
     */
    public function on404() {
        return [
            '#markup' => $this->t('The requested page could not be found.'),
        ];
    }

}