How do I capture an Ajax error in the data-ajax-failure function of my form in ASP.NET Core?

Question

I want to capture my error message in the data-ajax-failure portion of my form which uses unobtrusive ajax. It's easy to do this if the form is posted via a function which calls $ajax but I'm not doing that with this, I'm placing both my success and failure function calls in the form itself:

<form method="post" data-ajax-url="/Home/Process_Form" data-ajax="true" data-ajax-method="post" data-ajax-success="form_data_success" data-ajax-failure="form_data_fail">

The form post to a controller action called Process_Form

public IActionResult Process_Charterer(Charterer model) {        
    if (ModelState.IsValid)
    {
        bool safetyCheck = _chartererService.GetCharterers().Any(x => x.ChartererName.Contains(model.ChartererName));
        if (safetyCheck == true)
        {
            //Return something to the ajax failure
        }
        else
        {
            try
            {            
                _chartererService.InsertCharterer(model);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
    }
    return PartialView("_Window_Charterer");
}

The function should then capture the failure and do something with it, for example I'm going to show an alert for the user.

function form_data_fail(result) {
    //capture the error from the controller and do something with it.        
}

Can anyone explain how this is done with this particular setup?