Provide better error message for duplicate email

Administrators who tried to create an account with an email address which was already assigned to someone else get the less the awesome error message '1. unique'.

i18n errors code for ManagedSubject updated to be useful.

Closes #179.
Merge pull request #180 from ausaccessfed/bugfix/i18n-errors-unique-email-managed-subject

Provide better error message for duplicate email