storagenode/blobstore/filestore: add debug log to WalkNamespace when dir does not exist

This is particularly useful for monitoring the lazyfilewalker to
make sure it is not checking the wrong directory.

Updates https://github.com/storj/storj/issues/5349

Change-Id: I7e5fcfd4545ec4157d33a9225cd1bce607ccd154

storagenode/blobstore/filestore/dir.go

@@ -717,6 +717,7 @@ func (dir *Dir) walkNamespaceInPath(ctx context.Context, namespace []byte, path
 	openDir, err := os.Open(nsDir)
 	if err != nil {
 		if os.IsNotExist(err) {
+			dir.log.Debug("directory not found", zap.String("dir", nsDir))
 			// job accomplished: there are no blobs in this namespace!
 			return nil
 		}