local: fix partial directory read for corrupted filesystem

Before this change if a directory entry could be listed but not
lstat-ed then rclone would give an error and abort the directory
listing with the error

    failed to read directory entry: failed to read directory "XXX": lstat XXX

This change makes sure that the directory listing carries on even
after this kind of error.

The sync will be failed but it will carry on.

This problem was caused by a programming error setting the err
variable in an outer scope when it should have been using a local err
variable.

See: https://forum.rclone.org/t/sync-aborts-if-even-one-single-unreadable-folder-is-encountered/39653

backend/local/local.go

@@ -516,7 +516,7 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 								continue
 							}
 						}
-						err = fmt.Errorf("failed to read directory %q: %w", namepath, fierr)
+						fierr = fmt.Errorf("failed to get info about directory entry %q: %w", namepath, fierr)
 						fs.Errorf(dir, "%v", fierr)
 						_ = accounting.Stats(ctx).Error(fserrors.NoRetryError(fierr)) // fail the sync
 						continue