Handle race condition

If you're bootstrapping a node and run into an error where it doesn't finish successfully, you will have `Not checked in` under `Last Check-in`. The node in question will then return `nil` for `node["ohai_time"]`.

I'm making an assumption here that users will want to be alerted/reminded that their `knife bootstrap` failed so they will go ahead and delete the client+node from the chef server or reprovision it.