[fix] fixed https://github.com/osiam/addon-self-administration/issues/63

- when the password reset was already done by the user a FORBIDDEN with an error
  description will be returned
`SelfAdministrationHelper` would be much better than the current name. alternatively, you can extract the methods to separate classes and remove the whole `RegistrationHelper` class at all.

btw: you can delete the method `RegistrationHelper#replaceOldPrimaryMail`, it's not used anywhere (except in the corresponding unit test).