Catch BadWindow errors from incorrect _NET_SUPPORTING_WM_CHECK values

Developers testing Servo ran into this issue: https://github.com/mozilla/servo/issues/1521

In general, it's a bad idea to assume anything about data from X properties, as they can be read and written by any program on any Window. Relevant documentation re: XSynchronize is at <http://tronche.com/gui/x/xlib/event-handling/protocol-errors/synchronization.html>.