Fixed Bug #7621

Magicmushroom poison now properly cancels skills being cast upon activating one of its own, also modified unit_skillcastcancel's error message upon timer deletion failure so that it becomes easier to debug in the future.
http://hercules.ws/board/tracker/issue-7621-erro-map-server-waraitake/
Thanks to Beret

Signed-off-by: shennetsind <ind@henn.et>