How to set error messages in Klaviyo Popup?

I implemented the Klaviyo Popup in the way that is explained at https://help.klaviyo.com/hc/en-us/articles/115005249548-Add-and-Customize-a-Legacy-Popup. How can I configure the error messages when the form does not validate? For example in the documentation I found this:

<script type="text/javascript" src="//www.klaviyo.com/media/js/public/klaviyo_subscribe.js"></script>
<script type="text/javascript">
    KlaviyoSubscribe.attachModalSignUp({
        list: 'LIST_ID',
        delay_seconds: 10.5,
        content: {
            clazz: ' klaviyo_modal_LIST_ID',
            header: 'Join our Newsletter!',
            subheader: 'Get the latest and greatest news from us.',
            button: 'Subscribe Me!',
            success: 'Thanks! Check your email for a confirmation.',
            extra_fields: ["$first_name", "$last_name"],
            styles: '.klaviyo_modal.klaviyo_modal_LIST_ID { font-family: "Helvetica Neue", Arial}.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_header { color:#222;}.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_subheader { color:#222;}.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_submit_button,.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_submit_button span { background-color:#0064cd; background-image: none; border-radius: 2px;}.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_submit_button:hover,.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_submit_button span:hover { background-color:#0064cd; background-image: none; }.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_inner,.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_fieldset .klaviyo_field_group input[type=text],.klaviyo_modal.klaviyo_modal_LIST_ID .klaviyo_fieldset .klaviyo_field_group input[type=email] { border-radius: 2px;}'
        }
    });
</script>

Notice how in the documentation they include this part:

success: 'Thanks! Check your email for a confirmation.',

I was expecting to see something like this for error messages:

error_message: 'My custom error message.',

Is there a way to configure custom error messages? Thank you.

When reading this Klaviyo's blog entry at https://www.klaviyo.com/blog/new-klaviyo-popup-builder, it is my impression that they want to focus on this New Klaviyo Form Builder, which is great for people who need something easier than writing HTML/CSS. It would not surprise me if Klaviyo does not have a way to customize error messages using the "Legacy Popup", as they call it at https://help.klaviyo.com/hc/en-us/articles/115005249548-Add-and-Customize-a-Legacy-Popup, even though to me it should not be a "legacy" way to build pop-ups in Klaviyo.