A client sent us this tip to share with everyone.

In the application directory are two files, maintenance.html and offline.html. Looking at offline.html it shows there's room for an error code. It would be my guess that that html is a catch-all for errors, including 500s

So, if you know some html, you could edit those pages (make a copy of them first in case you need to revert changes) and add your own custom message or even add some css styling. Please note that you would want to keep from changing the maintenance code itself as it would block people from entering the code if that's messed with. Also, these would be overwritten on upgrades so you would want a copy of your changes as well when you upgrade.