Added GearmanTask to GearmanClientCallbackExceptionEvent.

The ::setExceptionCallback provides the GearmanTask to the function.
This was tested with PECL Gearman 1.1.2

Successfully tested with the event:
gearman.client.callback.exception

http://www.php.net/manual/en/gearmanclient.setexceptioncallback.php
It doesn't mention that the task is provided at current time, however I have tested that it is.

https://github.com/mmoreram/GearmanBundle/issues/59