Checks if task's gio.Cancellable has been cancelled, and if so, sets task's error accordingly and completes the task (see Task.returnPointer for more discussion of exactly what this means).
TRUE if task has been cancelled, FALSE if not
2.36
Checks if task's gio.Cancellable has been cancelled, and if so, sets task's error accordingly and completes the task (see Task.returnPointer for more discussion of exactly what this means).