Complete registration even if 'email' is not returned

The Facebook API sometimes does not return the 'email' field. Reasons are detailed [here](https://developers.facebook.com/bugs/298946933534016/) and include:

* No Email address on account
* No confirmed email address on account
* No verified email address on account
* User entered a security checkpoint which required them to reconfirm their email address and they have not yet done so
* Users's email address is unreachable

This commit ensures that registration can complete in this situation.