You can customise your error pages by adding the custom .htaccess.

To find this login to https://panel.myownfreehost.net/ as Administrator, then click Customisation, then click Default Htaccess.

In there you can add the code which will be displayed for the errors generated by your free hosting clients which register after the time you set the code in the reseller panel, clients who registered before you added the code will not use the custom error pages.