[Twisted-Python] A pseudo-deferred class that can be canceled

[Terry Jones]

Argh..... sorry, there was a trivial cut & paste error in the code I just
posted.  The errback method should of course be:

    def errback(self, fail=None):
        if not self._called:
            self._called = True
            defer.Deferred.errback(self, fail)

Terry