Minor refactor in cluster.go. Also improve error message.
Minor refactor in cluster.go. Also improve error message.

kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,abbytiz/minikube,aaron-prindle/minikube,cu12/minikube,cu12/minikube,skipjaq/minikube,dlorenc/minikube,kubernetes/minikube,aaron-prindle/minikube,dlorenc/minikube,dongsupark/minikube,dongsupark/minikube,dlorenc/minikube,dlorenc/minikube,endocode/minikube,dongsupark/minikube,kubernetes/minikube,kubernetes/minikube,abbytiz/minikube,dlorenc/minikube,skipjaq/minikube,r2d4/minikube,r2d4/minikube,cu12/minikube,abbytiz/minikube,warmchang/minikube,mikegrass/minikube,kubernetes/minikube,dlorenc/minikube,dims/minikube,warmchang/minikube,dalehamel/minikube,warmchang/minikube,aaron-prindle/minikube,r2d4/minikube,mikegrass/minikube,endocode/minikube,warmchang/minikube,warmchang/minikube,endocode/minikube,dims/minikube,dlorenc/minikube,kubernetes/minikube,mikegrass/minikube,skipjaq/minikube,dims/minikube,dlorenc/minikube,warmchang/minikube,dalehamel/minikube,warmchang/minikube