The return code of XGetWindowAttributes() indicates whether an error
was encountered or not. There is no need to specifically check the error
trap.

The trap around XAddToSaveSet() was superfluous. We have a global error
trap to ignore any errors here, and there is no need to XSync() as GDK
will later ignore the error asynchronously if one is raised.

Also move common error exit path to an error label.

https://bugzilla.gnome.org/show_bug.cgi?id=721345