Add a warning if scancode lookup failed
Add a warning if scancode lookup failed

This helps tracking some send_keys() issues, such as
https://bugzilla.gnome.org/show_bug.cgi?id=686170

freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk