v1.1.0: Exit immediately if kubeconfig file not found

If KUBECONFIG file not found, the tool will exit immediately,
give user a faster chance to fix the problem, and also avoid any potential issues
if the user command won't deal well with wrong/missing KUBECONFIG file.

And gk8s :local will try the default configuration $HOME/.kube/config
(so that any existing KUBECONFIG environment variables won't affect
the behavior of the command.)