In the registration resolvers, it makes sense to immediately resolve if
an email address has been found (because you can re-send the
registration email).
In this case, we use the helper method only to trigger the `UserInputError`.
It might be that people try to register email addresses that they don't own. Then if the actual owner tries to add this email address, she should not get a unique constraint violation. Instead the email will be re-used.
Is this a security issue? Because we re-use the nonce? 🤔