wc-custom-stripe-js-callback.js

function my_stripe_custom_callback( data ) {
	alert( data.response.error.message );
	data.form.unblock();
}

wc-custom-stripe-js-callback.php

<?php
/**
 * Plugin Name: Custom WC Stripe JS callback.
 * Description: Example to override error response handler for WC Stripe
 * Version: 1.0.0
 * Author: WooThemes
 * Author URI: https://woothemes.com
 */

// Pass my function name that handles error response handler.
add_filter( 'wc_stripe_params', function( $params ) {
	$params['error_response_handler'] = 'my_stripe_custom_callback';
	return $params;
} );

// Enqueue custom JS callback.
add_action( 'wp_enqueue_scripts', function() {
	if ( ! is_checkout() ) {
		return;
	}

	wp_enqueue_script( 'wc-custom-stripe-js-callback', plugins_url( 'wc-custom-stripe-js-callback.js', __FILE__ ), array( 'woocommerce_stripe'  ), '1.0.0', true  );

} );