When an error occurs with nested traps in place, only the innermost
trap should have the error code stored in it; outer traps are
shielded by the inner trap.
https://bugzilla.gnome.org/show_bug.cgi?id=629608
* don't lose track of previous X error handler
if nested traps are pushed
* free any remaining traps when display
is finalized
Test will fail unless bug 630032 is closed so
gdk_display_close() works.
https://bugzilla.gnome.org/show_bug.cgi?id=630033
