Bail out if trying to start a cluster that was not created

[krnowak]

Currently running kube-spawn destroy; kube-spawn start … will always result in a timeout when waiting for machines to be brought up. kube-spawn should rather fail much earlier, after noticing that the cluster was not created yet and telling to use kube-spawn create … first (or just use kube-spawn up …).

[dongsupark]

+1. Yeah, it should be more friendly.