MastodonAPI.MastodonAPIController: Return a 404 when we fail to get a list

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

@@ -905,7 +905,10 @@ def get_list(%{assigns: %{user: user}} = conn, %{"id" => id}) do
       res = ListView.render("list.json", list: list)
       json(conn, res)
     else
-      _e -> json(conn, "error")
+      _e ->
+        conn
+        |> put_status(404)
+        |> json(%{error: "Record not found"})
     end
   end