scope launch doesn't tell you if you got it right

[jml]

scope launch looks like it always works:

$ ./scope launch aouauenthsonteuhaosue.com
1819f0be0e5ea962250c3ec5f0098f5737ffb651a2165b09e0640c48f2c96852
Weave Scope is reachable at the following URL(s):
  * http://192.168.1.83:4040/
  * http://172.19.0.1:4040/
  * http://10.32.0.1:4040/
  * http://172.18.0.1:4040/

$ ./scope launch
2d612f472a71f51a082349d87522835f793b00c7773bbd3d1b506a0341968a23
Weave Scope is reachable at the following URL(s):
  * http://192.168.1.83:4040/
  * http://172.19.0.1:4040/
  * http://10.32.0.1:4040/
  * http://172.18.0.1:4040/

$ ./scope launch ===============
dc6bb878ea4fa57c55451fee4326b427f0c4cfb471c20471c8722b64b9dc5412
Weave Scope is reachable at the following URL(s):
  * http://192.168.1.83:4040/
  * http://172.19.0.1:4040/
  * http://10.32.0.1:4040/
  * http://172.18.0.1:4040/

Each of these cases has exit code 0.

When you browse to the given URLs, there is no indication that the input was erroneous.

Thie lack of good feedback hinders a user's ability to learn scope by exploration.

[jml]

Increases impact of #1556 and #1557.