Write cli errors to stderr

This explicitly writes errors to stderr, for the benefit of the
document plugin subcommand.  This is needed so that errors are displayed
to end users of kustomize, and can be used to debug manifest issues.
When sent to stdout, the error message gets swallowed/hidden by kustomize.

Change-Id: I49b668e5e71fbe4662fe3aec7bad4ebccadebd64

main.go

@@ -29,7 +29,7 @@ func main() {
 	}
 
 	if err := rootCmd.Execute(); err != nil {
-		fmt.Fprintln(os.Stdout, err)
+		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
 }