[NbtkStyle] mark error string as translatable
[NbtkStyle] mark error string as translatable

Error messages in GErrors should be translatable

jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx