[metadata-store] Provide failed keys for set_metadata() callback.
[metadata-store] Provide failed keys for set_metadata() callback.

vrutkovs/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,kyoushuu/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,kyoushuu/grilo-plugins,grilofw/grilo-plugins,MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,kyoushuu/grilo-plugins,MathieuDuponchelle/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins