Add descriptive error for Facebook auth when email is missing

A user can configure their privacy settings in Facebook to hide their email address, in which case an email is not returned from a call to /me.  When automatic provisioning is turned on, this causes user creation to fail.  I've added a check to rest.makeSessionFromEmail which will log a specific error message in this case.  No functional changes were made to the gateway.

In the long term, I think it would be a better idea to use the Facebook UID instead of the email address for the username.