rest: Don't return error if listing non-existent directory

When transferring a repository from S3 to, for example, a local disk
then all empty folders will be missing.

When saving files, the missing intermediate folders are created
automatically. Therefore, missing directories can be ignored by the
`List()` operation.

internal/backend/rest/rest.go

@@ -327,6 +327,11 @@ func (b *Backend) List(ctx context.Context, t restic.FileType, fn func(restic.Fi
 		return errors.Wrap(err, "List")
 	}
 
+	if resp.StatusCode == http.StatusNotFound {
+		// ignore missing directories
+		return nil
+	}
+
 	if resp.StatusCode != 200 {
 		return errors.Errorf("List failed, server response: %v (%v)", resp.Status, resp.StatusCode)
 	}