Bug 19579: Do not confirm the registration if email already used - PatronSelfRegistra...

Bug 19579: Do not confirm the registration if email already used - PatronSelfRegistrationEmailMustBeUnique

If PatronSelfRegistrationVerifyByEmail and
PatronSelfRegistrationEmailMustBeUnique are set, it should not be
possible to register twice with the same email.
However the test is made on already created patron cards when the
registration is done.
Which means it is possible to register several times with the same
email address and click on the registration link to finalise the
registration.

This patch adds a test when the registration link is clicked and display
the "Registration invalid" generic message if the same email is used

Test plan:
1. Patron submits self registration form using the same email address 3
times
2. Patron receives 3 verification emails
3. Patron clicks on 3 verify token URLs
=> Only the first registration should succeed, the 2 others must fail

Maybe we should display a more specific message?

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>