[Plugin: Contact Form 7] On fail?

Hi,

I’m wondering if there’s a way to display an error message when all required fields of the form haven’t been filled in correctly.
I currently use the on_sent_ok: "alert(' ');"when everything has been filled in correctly but I’d really like an on_fail or something, too

Thanks in advance!

http://wordpress.org/extend/plugins/contact-form-7/