How to display error message only in OnFailure of Ajax.BeginForm in MVC3?

I have this simple ajax form in my MVC3 app, that sends email message:

@using (Ajax.BeginForm("SendMessage", new AjaxOptions { 
    LoadingElementId = "wait", 
    OnSuccess = "loadMessages", 
    OnFailure = "showError" }))
{
    <input type="text" name="message" placeholder="Message" />
    <input type="submit" name="submit" value="Send" />
}

If the action fails to send a message, it throws the exception. The exception is handled and displayed in the showError(error){} javascript function:

function showError(error) { 
    $("#error").text(error.responseText);
};

The problem is: error.responseText is the html dump of the entire error page.

I need to display only the exception error message (whatever is in ex.Message in MVC3).

The implementation of "public ActionResult SendMessage(string message) { }" is irrelevant.

I need to know how to display the exception's message only in showError(error) javascript handler.

Thanks a bunch.

    [HttpPost]
    public ActionResult SendMessage(string message)
    {
        MailMessage mail = new MailMessage();
        SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
        mail.From = new MailAddress("sender@gmail.com");
        mail.Subject = "Some Message";
        mail.Body = message;
        mail.To.Add("recepient@gmail.com");
        SmtpServer.Send(mail);
        return null;
    }