net/http/httputil: ReverseProxy should use StatusBadGateway if backend can't be connected #9864
Description
When using net/http/httputil ReverseProxy where the upstream server is, for example, refusing connections the HTTP status code returned is StatusInternalServerError. It would be more appropriate (and helpful to the client) to return StatusBadGateway.
Looking at the underlying code for http.Transport, it looks like there is no way to differentiate between the two classes of error at the moment.