The below code snippet explains how to convert jQuery AJAX response.responseText string to JSON object in the jQuery.ajax error handler
<script type="text/javascript">
    $.ajax({
        type: "POST",
        url: "<URL>",
        data: "{}",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (response) {
 
        },
        error: function (response) {
            var responseTextObject = jQuery.parseJSON(response.responseText);
        }
    });
</script>
 
Above I have used jQuery.parseJSON method to convert the jQuery AJAX response.responseText string to JSON object