Discard task on exception in decode step

[acoover]

In worker_handler, we should discard jobs that raise exceptions when attempting to decode

https://github.com/Yelp/python-gearman/blob/master/gearman/worker_handler.py#L134

[svisser]

Related to this: a subclass of gearman.DataEncoder should be able to indicate that it couldn't decode the message. I'm now using a hack in decode to return something (ideally I should be able to raise an exception or otherwise indicate decoding could not be done):

    @classmethod
    def decode(cls, decodable_string):
        try:
            return json.loads(decodable_string)
        except (TypeError, ValueError):
            return InvalidJobDictionary()