Add more verbose error for ECONNREFUSED

If the tunnel server can be reached (at e.g. http://localtunnel.me/?new) but the tunnel (e.g. `grpi.localtunnel.me:44827`) can't actually be established, the client currently gets stuck in a loop retrying forever with no indication as to what's wrong. This doesn't fix the loop, since it does seem desirable to retry forever, but logs:

    [Error: connection refused: localtunnel.me:44827 (check your firewall settings)]

IMO, this closes #24.