better error handling for invalid weave status sub-command

[rade]

instead of showing '404 page not found'

Fixes #1328.

[awh]

This is printing an unwanted newline when the sub-command output is empty:

vagrant@vagrant-ubuntu-vivid-64:~/weave$ ./weave status peers
da:67:55:44:54:81(vagrant-ubuntu-vivid-64)
vagrant@vagrant-ubuntu-vivid-64:~/weave$ ./weave status dns

vagrant@vagrant-ubuntu-vivid-64:~/weave$ ./weave status connections

vagrant@vagrant-ubuntu-vivid-64:~/weave$ ./weave status targets

vagrant@vagrant-ubuntu-vivid-64:~/weave$

[rade]

This is printing an unwanted newline when the sub-command output is empty

Fixed.