Fix returning errors from snapshotter.Prepare

In the case of an error when calling snapshotter.Prepare we would return
nil. This change fixes that and returns the error from Prepare all the
time.

Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>

daemon/containerd/image_snapshot.go

@@ -51,9 +51,6 @@ func (i *ImageService) PrepareSnapshot(ctx context.Context, id string, parentIma
 	}
 
 	s := i.client.SnapshotService(i.StorageDriver())
-	if _, err := s.Prepare(ctx, id, parent); err == nil {
-		return err
-	}
-
-	return nil
+	_, err = s.Prepare(ctx, id, parent)
+	return err
 }