Make WalkDir return errors (#19677)

If used, 'opts.Marker` will cause many missed entries since results are returned 
unsorted, and pools are serialized.

Switch to fully concurrent listing and merging across pools to return sorted entries.

cmd/utils.go

@@ -1136,3 +1136,9 @@ func sleepContext(ctx context.Context, d time.Duration) error {
 	}
 	return nil
 }
+
+// helper type to return either item or error.
+type itemOrErr[V any] struct {
+	Item V
+	Err  error
+}