dockerng: use error from modules.dockerng in states' __virtual__
dockerng: use error from modules.dockerng in states' __virtual__

This improves the return value for errors in __virtual__ by using the
information from `modules.dockerng`.

Ref: https://github.com/saltstack/salt/issues/27643#issuecomment-148942821

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt