If someone tries to view a page on your website which doesn't exist, they will see page while looks like this:
We call this page a 404 page. You can create your own custom 404 page. Simply create a file called 404.html in the same directory as your index.html. Recompress your website, and upload. Your custom page will now be shown instead of the default one.
If you run into difficulties, shoot us an email and we'll help you get set up!