Emit IP address on network errors

[csilvers]

When running fastly compute serve (for a compute@edge service, though we see the same behavior for our vcl service), when I make a request that needs to hit a backend, and the backend is not running, I'll see this in the logs:

Error: hyper::Error(Connect, ConnectError("tcp connect error", Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }))

The problem is that we have many backends, and this gives no indication which backend is causing the problem. (Or even if it's a backend at all!)

Would it be possible to augment this error message to say the IP+port of the server that hyper failed to connect to?