Membership signup bug - missing sanity check - (minor)

If the user is stupid, and enters an invalid email address, WordPress chokes on the signup and a user readable error is not thrown. A simple regex would fix the problem. This is low priority because WordPress prevents the malformed user from being created, it just doesn't explain why.

This is only an issue if the user attempts to enter any string that is not formatted as an email address.

I know...I'm a pain in the ass.