A 502 error is a server fault at their end Not yours And if you do want to email the person about this error If its a small 1 man site There'll be I imagine a Cached Google Page where you may be able to find thst address

Only other way of comtacting is to post an email addressed to Webmaster @www.siteaddress.com (replace with sitename) or whatever its called?

But 5xx messages are Not on your computer Its a breakdown at the other end
www.checkupdown.com/status/E502.html
Who knows It might be up and running again now? Check that before doing anything..