TwitterAPI.UtilController: fix dialyzer errors

lib/pleroma/web/twitter_api/controllers/util_controller.ex:300:pattern_match
The pattern can never match the type.

Pattern:
{:error, :no_such_alias}

Type:
{:not_found, nil}

lib/pleroma/web/twitter_api/controllers/util_controller.ex:304:pattern_match
The pattern can never match the type.

Pattern:
{:error, _error}

Type:
{:not_found, nil}

lib/pleroma/web/twitter_api/controllers/util_controller.ex

@@ -319,7 +319,7 @@ defp find_user_by_nickname(nickname) do
     user = User.get_cached_by_nickname(nickname)
 
     if user == nil do
-      {:not_found, nil}
+      {:error, :not_found}
     else
       {:ok, user}
     end