Facebook login: no email => permanent failure

Now that we use the Graph API 2.0, users have the option to refuse to give some information when registering through Facebook.
The email address is the thing they can refuse to give.

We crash there, since we need that email, and display a "social login generic error".

We should display a more elegant error which should tell the users how to fix that: they should remove the app and give us the allowance. Maybe we should also indicate that they can register separately using email and that their account can get merged.