Replace Bad Gateway page on server with a more user-friendly note

[fbanados]

trafficstars

When the server has some issue (e.g. it stops processing requests) it generally produces a 502 bad gateway error.

It would be useful to have some feedback page saying that it's a problem on the server side (something like "Woops, there's a problem with the altlab server!") and add a contact email for when these errors happen.