Bug 20139: Improve MARC mapping translatable strings

Bug 20139: Improve MARC mapping translatable strings

- Allow translating one error message
- Use string formatting with placeholders instead of concatenation

Test plan:

1) Go to Home -> Administration -> Koha to MARC Mapping
2) Click on the "Add" button to add a new mapping
3) Note the popup text looks correct
4) Enter something invalid
5) Note the error message
6) Click on any of the "Remove" buttons to remove a mapping
7) Note the popup text looks correct
8) Update, translate the new msgid's, and install a language
9) Repeat 1-7, note how the message are in the correct
   language

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>