Serve more useful error page when server has not booted

[raxod502]

Currently if the supervisor process has not successfully booted a server container, it just serves a 502 response with an empty body. This is a result of the default behavior of httputil.NewSingleHostReverseProxy used here: https://github.com/raxod502/riju/blob/0992a4dc905474591a43aaf5c65f1ba1db380ce2/supervisor/src/main.go#L587-L588

Ideally we could at least serve a 502 with an error message that says the server has not booted.