kubeadm: Check for GetClientset error
kubeadm: Check for GetClientset error

`elevateKubeSystemPrivileges` was using `GetClientset` function
and getting the err object from, but err wasn't checked.

dlorenc/minikube,aaron-prindle/minikube,warmchang/minikube,dlorenc/minikube,warmchang/minikube,aaron-prindle/minikube,dlorenc/minikube,warmchang/minikube,kubernetes/minikube,aaron-prindle/minikube,warmchang/minikube,dalehamel/minikube,dims/minikube,dlorenc/minikube,kubernetes/minikube,dalehamel/minikube,kubernetes/minikube,kubernetes/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,dlorenc/minikube,warmchang/minikube,dlorenc/minikube,kubernetes/minikube,dlorenc/minikube,dalehamel/minikube,warmchang/minikube,dims/minikube,dims/minikube