transmogrifier: avoid crashes on non-validation Delte errors

Happens e.g. for duplicated Deletes.
The remaining tombstone object no longer has an actor,
leading to an error response during side-effect handling.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -572,6 +572,12 @@ defp handle_incoming_normalised(
         else
           _ -> e
         end
+
+      {:error, _} = e ->
+        e
+
+      e ->
+        {:error, e}
     end
   end