fix: handle meta backup offset errors safely (#9818)

* fix: log meta backup offset errors

* fix: log meta backup offset errors

* fix: exit on meta backup offset errors

Exit with a non-zero status when the initial metadata backup offset cannot be persisted.

Classify offset-read failures during streaming so the backup process exits instead of retrying forever, allowing supervisors to restart and bootstrap from a missing checkpoint.

* meta backup: read offset in the loop, drop offset error type

Reading the saved offset inside the retry loop makes an offset read
failure a clean exit and a stream error a retry, without a typed error
to tell them apart. streamMetadataBackup now takes the start time.

---------

Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/command/filer_meta_backup.go

@@ -115,15 +115,19 @@ func runFilerMetaBackup(cmd *Command, args []string) bool {
 			glog.Errorf("traverse meta data: %v", err)
 			return true
 		}
-		glog.V(0).Infof("metadata copied up to %v", startTime)
 		if err := metaBackup.setOffset(startTime); err != nil {
-			startTime = time.Now()
+			glog.Exitf("set metadata backup offset: %v", err)
+			return true
 		}
+		glog.V(0).Infof("metadata copied up to %v", startTime)
 	}
 
 	for {
-		err := metaBackup.streamMetadataBackup()
+		startTime, err := metaBackup.getOffset()
 		if err != nil {
+			glog.Exitf("read metadata backup offset from %s: %v", *metaBackup.filerAddress, err)
+		}
+		if err := metaBackup.streamMetadataBackup(startTime); err != nil {
 			glog.Errorf("filer meta backup from %s: %v", *metaBackup.filerAddress, err)
 			time.Sleep(1747 * time.Millisecond)
 		}
@@ -177,12 +181,8 @@ var (
 	MetaBackupKey = []byte("metaBackup")
 )
 
-func (metaBackup *FilerMetaBackupOptions) streamMetadataBackup() error {
+func (metaBackup *FilerMetaBackupOptions) streamMetadataBackup(startTime time.Time) error {
 
-	startTime, err := metaBackup.getOffset()
-	if err != nil {
-		startTime = time.Now()
-	}
 	glog.V(0).Infof("streaming from %v", startTime)
 
 	store := metaBackup.store