BUG:371523 Do not crash on opening Gimp 2.9 files

Krita cannot load Gimp 2.9 files (and never will be, probably,
because xcf-tools isn't maintained anymore and the file format
is un(der)documented). But we shouldn't crash and burn trying
to load them anyway.

This adds two new strings for translation, I'm afraid :-(

CCMAIL:kde-i18n-doc@kde.org