'Called by a Job object to tell that an exception occured
during the processing of the function. The object will become
ready but not successful. The collector\'s notify_ready()
method will be called, but NOT the callback method'

def _set_exception(self):

    assert (not self.ready())
    self._data = sys.exc_info()
    self._success = False
    self._event.set()
    if (self._collector is not None):
        self._collector.notify_ready(self)
