time: return informative errors when failing to load timezone data.

If we cannot load timezone information for a reason other than the
zoneinfo file not existing, return it since that will be much more
useful in debugging failures than "unknown time zone XYZ".

Fixes #9723.

Change-Id: I3aa5774859cec28e584d16bcc1fef0705d95288c
Reviewed-on: https://go-review.googlesource.com/3984Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/sys_unix.go

@@ -74,3 +74,5 @@ func preadn(fd uintptr, buf []byte, off int) error {
 	}
 	return nil
 }
+
+func isNotExist(err error) bool { return err == syscall.ENOENT }

src/time/zoneinfo_plan9.go

@@ -148,11 +148,12 @@ func initLocal() {
 }
 
 func loadLocation(name string) (*Location, error) {
-	if z, err := loadZoneFile(runtime.GOROOT()+"/lib/time/zoneinfo.zip", name); err == nil {
+	z, err := loadZoneFile(runtime.GOROOT()+"/lib/time/zoneinfo.zip", name)
+	if err != nil {
+		return nil, err
+	}
 	z.name = name
 	return z, nil
-	}
-	return nil, errors.New("unknown time zone " + name)
 }
 
 func forceZipFileForTesting(zipOnly bool) {

src/time/zoneinfo_unix.go

@@ -74,11 +74,17 @@ func initLocal() {
 }
 
 func loadLocation(name string) (*Location, error) {
+	var firstErr error
 	for _, zoneDir := range zoneDirs {
 		if z, err := loadZoneFile(zoneDir, name); err == nil {
 			z.name = name
 			return z, nil
+		} else if firstErr == nil && !isNotExist(err) {
+			firstErr = err
 		}
 	}
+	if firstErr != nil {
+		return nil, firstErr
+	}
 	return nil, errors.New("unknown time zone " + name)
 }

src/time/zoneinfo_windows.go

@@ -260,11 +260,12 @@ func initLocal() {
 }
 
 func loadLocation(name string) (*Location, error) {
-	if z, err := loadZoneFile(runtime.GOROOT()+`\lib\time\zoneinfo.zip`, name); err == nil {
+	z, err := loadZoneFile(runtime.GOROOT()+`\lib\time\zoneinfo.zip`, name)
+	if err != nil {
+		return nil, err
+	}
 	z.name = name
 	return z, nil
-	}
-	return nil, errors.New("unknown time zone " + name)
 }
 
 func forceZipFileForTesting(zipOnly bool) {