Check HTTP Response status when trying to download minikube.iso

Check the response so that we don't actually download an error page or
something other than the ISO we're looking for.

pkg/minikube/cluster/cluster.go

@@ -308,6 +308,8 @@ func (m *MachineConfig) CacheMinikubeISOFromURL() error {
 	response, err := http.Get(m.MinikubeISO)
 	if err != nil {
 		return err
+	} else if response.StatusCode != http.StatusOK {
+		return fmt.Errorf("Received %d response from %s while trying to download minikube.iso", response.StatusCode, m.MinikubeISO)
 	} else {
 		out, err := os.Create(m.GetISOCacheFilepath())
 		if err != nil {