kubectl: Improve default kubectl behavior when it doesn't know what cluster to talk to

At minimum, we need to improve the error message: The connection to the server localhost:8080 was refused - did you specify the right host or port?

We might also want to consider not using localhost as the default, though we want to keep it aligned with our single-node dev cluster plans. kubernetes/kubernetes#24106

cc @kubernetes/kubectl @dlorenc @vishh