girepository: Add GI_VFUNC_THROWS

Virtual functions can definitely throw an error.  Right now the
scanner omits the GError parameter for them and adds throws="1", but
g-ir-compiler ignores this.

https://bugzilla.gnome.org/show_bug.cgi?id=669332

girnode.h

@@ -206,6 +206,7 @@ struct _GIrNodeVFunc
   gboolean must_be_implemented;
   gboolean must_not_be_implemented;
   gboolean is_class_closure;
+  gboolean throws;
 
   char *invoker;