Allow git pushes to respond with an informative failure message

If a downstream dependency of builder is not reachable, `git push`es should respond with an informative error message to the user. Since #149 adds probes, and those probes check downstream dependencies, the behavior immediately after that PR is that builder will be shut down by Kubernetes.

The implementation to close this issue will likely be to move the actual health check logic into the builder. There of course may be others.

Ref https://github.com/deis/builder/pull/149#issuecomment-184861922