Gtk::PrintJob: Fix a call to Glib::throw_exception()
Gtk::PrintJob: Fix a call to Glib::throw_exception()

Glib::throw_exception(GError*) takes ownership of the GError.
SignalProxy_Custom_gtk_callback() must give a copy to throw_exception().

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm