Stop current process when UpdateWorkerActivity gives a 409 on started

The first call to update_activity, with started should prevent running process_element if the backend gives a 409.

We should have a

except ErrorResponse as e:
  if state == ActivityState.started and e.status == 409:
    logger.info("Skipping this element because it was already processed.")
    return False
return True

then

if self.update_activity() is True:
  self.process_element(...)