Detach of unattached container succeeds silently

[awh]

Fixes #596.

vagrant@vagrant-ubuntu-utopic-64:~/weave$ ./weave launch
a4eba9d1b0a3dd0fd12ac3ce032387e1528b51641f286cf446182168c7983625
vagrant@vagrant-ubuntu-utopic-64:~/weave$ C=$(docker run -d -ti ubuntu)
vagrant@vagrant-ubuntu-utopic-64:~/weave$ ./weave detach 10.2.0.1/24 $C
vagrant@vagrant-ubuntu-utopic-64:~/weave$ echo $?
0
vagrant@vagrant-ubuntu-utopic-64:~/weave$ sudo ./weave --local detach 10.2.0.1/24 $C
vagrant@vagrant-ubuntu-utopic-64:~/weave$ echo $?
0

[awh]

@rade updated as discussed.