The panel comes with a few custom error pages to get you started.
These can be found in the /panel/etc/static/errorpages/ folder.
You can add as many error pages as you wish as long as they are named ###.html (where ### is the error number: 403.html, 404.html, etc.) and they will be transferred to each client's hosting space when they create a domain or subdomain.
If you add more error pages after clients have created their sites, you will need to manually copy them into each user's /_errorpages/ folder.
You may also offer your user's an easy way to customize their own error pages or restore the default pages by using the 3rd party Error Pages module.
You can view a demo of one of the ways you can use/customize your error pages HERE.