Fail gracefully for invalid network names

[sanimej]

Currently a traceback is thrown if the passed network name doesn't exist.

smanohar@net16-1:~/code/go/src/github.com/sanimej/ssd$ docker run -v /var/run/docker.sock:/var/run/docker.sock -v /var/run/docker/netns:/var/run/docker/netns --net=host --privileged sanimej/ssd ov2
Verifying LB programming for containers on network ov2
Traceback (most recent call last):
  File "/ssd.py", line 179, in <module>
    check_network(sys.argv[1])
  File "/ssd.py", line 59, in check_network
    data = cli.inspect_network(nw_name, verbose=True)
  File "/usr/lib/python2.7/site-packages/docker/utils/decorators.py", line 35, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/docker/api/network.py", line 188, in inspect_network
    return self._result(res, json=True)
  File "/usr/lib/python2.7/site-packages/docker/api/client.py", line 220, in _result
    self._raise_for_status(response)
  File "/usr/lib/python2.7/site-packages/docker/api/client.py", line 216, in _raise_for_status
    raise create_api_error_from_http_exception(e)
  File "/usr/lib/python2.7/site-packages/docker/errors.py", line 31, in create_api_error_from_http_exception
    raise cls(e, response=response, explanation=explanation)
docker.errors.NotFound: 404 Client Error: Not Found ("network ov2 not found")