get accurate retval from ssh commands

- throw an 'exception' if any of the critical ssh commands fail
- this is complicated by chgrp root possibly failing if the root
  group does not exist and is instead wheel -- which we ignore
- feels like this is going down the wrong rabbit hole, i'd prefer
  to just chain the commands with && and drop the explict chown and
  chgrp entirely.
Merge pull request #4 from opscode/lcg/retval-paranoia

get accurate retval from ssh commands