Save error message on a custom field of person record if webhook failed?

Similar article: Adding Webhook Error log to email alert

As I answered on the other thread, there's no token for a failed response. You can capture the fact that there was an error (and filter by error codes) using Webhook Was Called.

Ifyou have a remote service that uses HTTP errors + data payloads to send responses (this isn't a good practice for any web service) then you have to put another service between Marketo and that service to translate appropriately.