docs: GtkBuilder: warn about unusual GError use
docs: GtkBuilder: warn about unusual GError use

GtkBuilder returns GError for _add_from_{file,resource,string}(),
implying that the user should be able to recover from these errors.
Mention in the docs that it's unreasonable to try to do this.

https://bugzilla.gnome.org/show_bug.cgi?id=679930

Adamovskiy/gtk,chergert/gtk,jessevdk/gtk,grubersjoe/adwaita,bratsche/gtk-,chergert/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,bratsche/gtk-,ebassi/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,jigpu/gtk,msteinert/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,jessevdk/gtk,jigpu/gtk,ebassi/gtk,Lyude/gtk-,Lyude/gtk-,ebassi/gtk,chergert/gtk,Lyude/gtk-,jessevdk/gtk,Lyude/gtk-,ebassi/gtk,msteinert/gtk,msteinert/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,ahodesuka/gtk,Lyude/gtk-,alexlarsson/gtk,chergert/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,chergert/gtk,jigpu/gtk,bratsche/gtk-,alexlarsson/gtk,chergert/gtk,ebassi/gtk,davidgumberg/gtk,Adamovskiy/gtk,bratsche/gtk-,grubersjoe/adwaita,ahodesuka/gtk,ebassi/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,jadahl/gtk,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jessevdk/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,jadahl/gtk,msteinert/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,ahodesuka/gtk,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,jessevdk/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jessevdk/gtk,jadahl/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,Lyude/gtk-,msteinert/gtk,Sidnioulz/SandboxGtk,chergert/gtk,Lyude/gtk-,Lyude/gtk-,Sidnioulz/SandboxGtk,alexlarsson/gtk,jessevdk/gtk,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita