How to run custom code on form submit error

For anyone else reading this, I figured out a way to make this work by continuously looking for the form's error field inside the iframe then calling my function to resize if it's present:

var foundFormError = false;
window.setInterval(
    function() {
        if (!foundFormError) {
            var ctas = document.querySelectorAll(`iframe[id^="cta-iframe"]`);
            ctas.forEach(cta => {
                var iframeContent = document.getElementById(cta.id).contentWindow.document;
                if (iframeContent.getElementsByClassName('field-error').length > 0) {
                    foundFormError = true;
                    // resize here!
                }
            });
        }
    },
    100);