Fix multi cidr error messages

[awh]

Fixes #568:

vagrant@ubuntu-14:~/weave$ ./weave run -ti ubuntu
Invalid CIDR: -ti
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave start foo bar
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave attach foo bar
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave detach foo bar
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave expose foo
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave hide foo
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24
vagrant@ubuntu-14:~/weave$ ./weave hide 10.0.0.1/24 foo
Invalid CIDR: foo
CIDR must of be of form <ip_address>/<routing_prefix_length>, e.g. 10.2.1.1/24

[awh]

Updated as discussed.