activitypub: fix error condition match

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -756,7 +756,7 @@ def fetch_object_from_id(id) do
            {:ok, activity} <- Transmogrifier.handle_incoming(params) do
         {:ok, Object.normalize(activity.data["object"])}
       else
-        {:reject, nil} ->
+        {:error, {:reject, nil}} ->
           {:reject, nil}
 
         object = %Object{} ->