Bug 19837: Add error feedback for duplicated or unfound cardnumbers

Bug 19837: Add error feedback for duplicated or unfound cardnumbers

To test:
1 - Enter some patrons on a list
2 - 'Enter multiple cardnumbers'
3 - Make sure to add some that are already in list, some that don't
    exist, and some new patrons
4 - Verify results are as expected with warnings

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>