The basic 404 error or file not found page serves two specific purposes. Firstly, to alert the user that the item they were expecting is not currently available. Secondly, to alert the webmaster that a file is missing or a link is targeting something incorrectly. In order to provide users with a page that will show up whenever a 404 error occurs you need to change the settings on your web server to target a specific page or contact your hosting company to do this for you.