Better support for error pages

[drspangle]

Is your feature request related to a problem? Please describe.

Right now, when a server that is being proxied via NPM isn't yet available, a 502 "bad gateway" error is returned. This is fine, it's the expected behavior, but it would be a lot nicer if it were possible to instead redirect to a nicer error page. This would be especially helpful if that page could include javascript to redirect back to the original page after a time, allowing services to come online (obviously this custom page would be written by the end user).

Describe the solution you'd like

I'd like to be able to specify the error pages for handling various errors, in particular 502. You should be able to go into a domain's settings and specify the redirection URL, and/or specify a custom static HTML file to use.

Describe alternatives you've considered

The only real alternative here is to manually edit the nginx config inside the container, which there is lots of documentation to show is possible, but that defeats the purpose of using NPM.

Additional context