Prevents users from logging in if email isn't shared in their Facebook account.

rule.js

function (user, context, callback) {

  if (context.connection === "facebook" && !user.email) {
    return callback(new UnauthorizedError("We can't access your email. Please enable sharing your e-mail from Facebook apps settings and re-try."));
  }
  return callback(null, user, context);
}