time: return ENOENT if a zoneinfo zip file is not found

Updates #20969

Change-Id: Ibcf0bf932d5b1de67c22c63dd8514ed7a5d198fb
Reviewed-on: https://go-review.googlesource.com/c/155538
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/zoneinfo_read.go

@@ -271,7 +271,7 @@ func get2(b []byte) int {
 func loadTzinfoFromZip(zipfile, name string) ([]byte, error) {
 	fd, err := open(zipfile)
 	if err != nil {
-		return nil, errors.New("open " + zipfile + ": " + err.Error())
+		return nil, err
 	}
 	defer closefd(fd)