How do I configure error pages for my Fleet when an error occurs?

Magento on Fleet makes use of Apache's .htaccess system where you can configure webserver directives.

If in your code you create a file called .htaccess in the Document Root You can add directives to it like:

ErrorDocument <error code> <path relative to document root>

Eg. To show 500.html when there is a 500 Internal Server Error you would add the following to your .htaccess file:

ErrorDocument 500 500.html

How do I show an error page if my Fleet is down?

If there are no healthy frontends available due to some issue with your website you can configure CloudFlare to serve requests instead.

See CloudFlare's documentation on how to use this feature: