core: annotate nullable error in callbacks
core: annotate nullable error in callbacks

Since those arguments are optional error given to callbacks, annotate
them as nullable.

https://bugzilla.gnome.org/show_bug.cgi?id=740186

GNOME/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,GNOME/grilo,MathieuDuponchelle/grilo,grilofw/grilo,MathieuDuponchelle/grilo,grilofw/grilo,jasuarez/grilo,jasuarez/grilo,grilofw/grilo