Exit with status 3 if no failures but unreachable.

Provide hints to playbook callers that a playbook execution had
unreachable vs failures. 2 == failures, 3 == no failures, but
unreachable hosts. 0 continues to be all good.
Merge branch 'exit-3-on-dark' of https://github.com/j2sol/ansible into j2sol-exit-3-on-dark