Bug 18367 - Fix untranslatable string from Bug 18264

Bug 18367 - Fix untranslatable string from Bug 18264

Restore display of message when no barcode found

To test:
1 - Go to course reserves
2 - Add an item to a course
3 - Try adding an invalid barcode - should get an alert
4 - Try force adding an item with itemnumber:
    http://localhost:8081/cgi-bin/koha/course_reserves/add_items.pl?op=lookpup&course_id=11&itemnumber=1234523546
5 - Should recieve an error

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>