time: improve error message for LoadLocation

Currently, when a tz file was being checked inside a zoneInfo dir,
a syscall.ENOENT error was being returned, which caused it to look
in the zoneinfo.zip file and return an error for that case.

We return a syscall.ENOENT error for the zip file case too, so that
it falls through to the end of the loop and returns an uniform error
for both cases.

Fixes golang#20969

Change-Id: If1de068022ac7693caabb5cffd1c929878460140
Reviewed-on: https://go-review.googlesource.com/121877
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/zoneinfo_read.go

@@ -364,7 +364,7 @@ func loadTzinfoFromZip(zipfile, name string) ([]byte, error) {
 		return buf, nil
 	}
 
-	return nil, errors.New("cannot find " + name + " in zip file " + zipfile)
+	return nil, syscall.ENOENT
 }
 
 // loadTzinfoFromTzdata returns the time zone information of the time zone

src/time/zoneinfo_test.go

@@ -5,6 +5,7 @@
 package time_test
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -36,6 +37,16 @@ func TestEnvVarUsage(t *testing.T) {
 	}
 }
 
+func TestBadLocationErrMsg(t *testing.T) {
+	time.ResetZoneinfoForTesting()
+	loc := "Asia/SomethingNotExist"
+	want := errors.New("unknown time zone " + loc)
+	_, err := time.LoadLocation(loc)
+	if err.Error() != want.Error() {
+		t.Errorf("LoadLocation(%q) error = %v; want %v", loc, err, want)
+	}
+}
+
 func TestLoadLocationValidatesNames(t *testing.T) {
 	time.ResetZoneinfoForTesting()
 	const env = "ZONEINFO"