Fail if no png files have been produced

This indicates that something has gone wrong.  There should be at least
one.

go/archive/archive.go

		exit("filepath.Glob", err)
	}
	if len(parts) == 0 {
		exit("filepath.Glob", fmt.Errorf("no matches"))
	}
	h := fmt.Sprintf(".archive/%x.html", buf)
	f, err = os.OpenFile(h, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0660)
	if err != nil {