Recover from commands executed with unknown credentials

We'll fall back to executing the command without authentication if the command fails in a very specific way. This is a sign the provided private key is unknown to the Jenkins master. This exception is commonly thrown the first time a Chef run enables authentication on the Jenkins master. This should also fix some cases of [JENKINS-22346](https://issues.jenkins-ci.org/browse/JENKINS-22346).

/cc @opscode-cookbooks/release-engineers