Bug 15574: Better wording for error message when adding a duplicate tag

Bug 15574: Better wording for error message when adding a duplicate tag

To test:

1) Apply patch
2) Go to a record (ie opac-detail.pl) and add tags that already exist with that record
3) Confirm new error message

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Rewording Ok for common patrons

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit 56dde11a4f39b8a9f151d7592a7869f20c8c5f2f)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>