Part of elisa.core.utils.cancellable_defer View In Hierarchy
see twisted.internet.defer.Deferred.
Cancel this deferred.
If the deferred is waiting on another deferred, forward the cancellation to the other deferred.
If the deferred has not yet been errback'd/callback'd, call the canceller function provided to the constructor. If that function does not do a callback/errback, or if no canceller function was provided, errback with CancelledError.
Otherwise, raise AlreadyCalledError.