Reworked cancelation handling, if the cancel handler isn't specified the OperationCanceledException will be handled by the error handler Any unhandled OperationCanceledException will cause the promise cancelation

using System;

namespace Implab {
    public interface ITaskController: IProgressHandler, ICancellable {
        bool IsCancelled {
            get;
        }

        event EventHandler Cancelled;
    }
}