javascript form - error when hitting enter

I am working on a landing page that has a contact form made with javascript. It connects to my Email Service Provider. If someone enters in the three fields correctly, the info passes to my ESP just as it should. However, if someone hits the "enter" key too early, they get taken to another page with an error message. I would like an alert to display instead, that says what fields the person needs to fill out before they will be able to submit.

The page is available here: http://smcu.com/promos/auto-lg-aw/

Thank you,