Update markup for xi18n* in failed-save message box

Summary:
When localizing strings for the failed-save message box two warnings
complain about 'b' and 'br' tags on the command line.
This is caused by wrong markup tags for `xi18n*` in `GvCore`.

FIXED-IN: 18.08.0

Test Plan:
1. Start Gwenview from `Konsole` or `xterm` in View or Browse Mode
2. {nav File > Save As} or {key Ctrl Shift S}
3. Try to save in a folder you don't have write permission (e.g. Root)
4. Watch console for (not appearing) tag warnings

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D12343