I'm wondering if Marketo recognizes users who received a form error.
Am I able to segment these users?
A validation error, or a network error?
If a session is already associated with a known lead, you can (via custom JS) log the fact they failed validation to Marketo.
If the session is anonymous and/or there's a network error, by definition you can't log these to Marketo, though in theory you could log to another analytics service such as GA, Looker, et al.