We have error pages that are standard and served for all sites on WordPress.com, and as a VIP customer you can override these defaults and specify your own custom error pages instead.
Setting it up is simple: all you need to do is add a custom error page (a simple HTML file) to your theme.
I.e., from the root of your theme: ~/error-docs/error-page.html
You can view an example here:
Once you’ve committed the file to your theme, you’ll need to open a ticket with our team to notify us, so we can make some associated configuration changes on our end.