Improve error messages

[neur0manc]

Let's improve the error messages. We should be able to give more distinct information to the user, why something might have failed. E.g.:

newKubeConfig := fetchKubeConfigFromProvider(clusterID)
if len(newKubeConfig.Clusters) == 0 || len(newKubeConfig.Users) == 0 {
	fmt.Fprintln(os.Stderr, "Error: Invalid kubeconfig")
	os.Exit(1)
}
c := newKubeConfig.Clusters[0]
u := newKubeConfig.Users[0]

As a user this is quite confusing because it looks like there's something wrong with his local ~/.kube/config, which isn't the case at all. Furthermore, there should be separate messages for both cases, since both seem to have different originating errors. Something like

• "cluster not found"
• "invalid user or token" (if we can tell which is the cause be even more detailed)

This should clear out some initial headache and make the first use even more impressive (because gscloud is already awesome).