Fix soong_zip printing warnings with --ignore_missing_files

We weren't defaulting args.Stderr to os.Stderr, so we had been calling
fmt.Fprintln with `nil`.

Test: soong_zip --ignore_missing_files -C test/ -f abc -o test.zip
Change-Id: I0421f4bfb07fa36edf86c68ffc847995777c8221