Tidy and improve dbloader log message output.

- Add traceback for unexpected exceptions using logging.exception().
- Remove pointless type(exception) as this would always return 'instance'.
Unlock current message queue element on shutdown.

- Add unlocking of current msg queue element to shutdown method so no
  locked elements left if shutdown called during long loading operation.
- To support first change, make current element reference an instance
  object so it can be queried by shutdown method.
- Correct string formatting conversion in log message.