> this is probably the weakest part of it.

This is not a 'part of' htmx. Htmx doesn't prescribe how you handle errors. You can easily respond with 400 or some other appropriate status code on error, and plug in to an htmx hook on the client side to handle it appropriately.

Eg here's how I handle form validation errors with a 422 response status: https://dev.to/yawaramin/handling-form-errors-in-htmx-3ncg