You can customise the error pages posted by the server. You hence have to create an .htaccess file at the root of your website (in the directory /public_html/).
This file will have to include the error code to be managed and the page to be shown in this case.
For example to manage the errors 404 and 500, your file has to include :
ErrorDocument 404 /404.html
ErrorDocument 500 /erreur500.html
These pages have to exist otherwise other errors will be generated.
In the above example, you need to have the pages 404.html and erreur500.html in the directory /public_html.