Trigger form validation?

Anyone know how to trigger the default form validation? I am creating a multi step form, and have a next button that displays an overview of the fields they have entered values into, and also show the submit button to actually submit the form. I would like to peform the form validation before sending to the overview step where they can then actually submit the form.

I know there is a function on_sent_ok, but is there something for if the form has validation errors?

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