Fix multiprocess connection

This PR is to fix : 
1. multiprocessing issues with database connection when sending queued email
2. Handle error on ``send_queued_email`` when no queued emails are available

I have a hard time to test this issue of multiprocess, please advise @selwin

especially on https://github.com/SeiryuZ/django-post_office/compare/fix-multiprocess-connection#L1R57