Bug 32426: Adjust error message of ERROR_login_exist

Bug 32426: Adjust error message of ERROR_login_exist

The reference to password might be confusing. You could have the
same password as another user.
The addition of 'could not create unique one' especially applies to
plugins that return an existing userid, like the very simple plugin
example on the Bugzilla report that just returns borrowers.email.

Test plan:
Covered by previous patch. Just a string change here.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>