Fix error when last sync is present but is invalid datetime

lib/frenzy_web/controllers/fervor/items_controller.ex

@@ -135,10 +135,12 @@ defmodule FrenzyWeb.Fervor.ItemsController do
       |> Repo.all()
 
     last_sync =
-      Map.get(params, "last_sync")
-      |> case do
-        s when is_binary(s) -> Timex.parse!(s, "{ISO:Extended:Z}")
-        _ -> nil
+      with s when is_binary(s) <- Map.get(params, "last_sync"),
+           {:ok, datetime} <- Timex.parse(s) do
+        datetime
+      else
+        _ ->
+          nil
       end
 
     {deleted_ids, upserted} =