gatecoin: added proper exception for order that has been already cancelled
gatecoin: added proper exception for order that has been already cancelled

gatecoin.js is already throwing OrderNotFound for cases where the order was actually existing at some point, so technically it is not "order not found". However this seems like a proper exception to throw along with the case with already canceled order and such.

ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt