Catch duplicate email addresses on user registration

[brookingcharlie]

When a user attempts to register with an email address that's associated with an existing account, they get a bare HTTP 500 response listing a ConstraintViolationException. Email addresses in OzTrack are considered to be a unique identifier for user accounts and used, e.g., to send password reset links. Check that address is unique and, if not, return a user-friendly error message.