Don't glog top level command error message

This is already printed to stderr through RootCmd.Execute().  We could
save to this the glog logs by supressing the RootCmd.Execute() error
output, but this displays a more user friendly error (i.e. no log
prepended info)

cmd/minikube/cmd/root.go

@@ -111,9 +111,7 @@ Please use --v=3 to show libmachine logs, and --v=7 for debug level libmachine l
 // Execute adds all child commands to the root command sets flags appropriately.
 // This is called by main.main(). It only needs to happen once to the rootCmd.
 func Execute() {
-	if err := RootCmd.Execute(); err != nil {
-		glog.Exitln(err)
-	}
+	_ = RootCmd.Execute()
 }
 
 // Handle config values for flags used in external packages (e.g. glog)