No error message

Issue Summary

When a visitor goes to http://www.example.com/mailchimp/subscribe and fills out the required fields but DOES NOT select a newsletter checkbox, they still receive this message:

"Thank you, you have been successfully subscribed."

It seems that selecting at least 1 newsletter to subscribe to should be required, otherwise the visitor should receive an error message (not a successful subscription message as is currently happening).

Thanks for helping with this!