Add more verbose error for ECONNREFUSED

If the tunnel server can be reached (at e.g. http://localtunnel.me/?new)
but the tunnel (to 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]
Merge pull request #27 from adammck/couldnt-establish-tunnel

Add more verbose error for ECONNREFUSED