you can set a custom 404 error page via a .htaccess file very easily. This will allow you to set a good looking error page insted of default boring page.
(1)create a page at your root directory name 404.php and design it.
(2)create a .htaccess file at your root directory or if you have already then open it
(2)add following code in this file
ErrorDocument 404 404.php