Improves exceptions logging

While using [ui/django-rq](https://github.com/ui/django-rq) and `django.utils.log.AdminEmailHandler`, I faced two issues:

- `UnpickleError` wasn't caught, and therefore I didn't get any mail when incorrect arguments were passed to RQ;
- Mails were poorly formatted.

At first I thought it was coming from django-rq, but after digging I realized this was coming from RQ.  This pull request fixes these two issues.
(No problem with the delay ;) )
`log.exception` is better since it adds a traceback.
Python docs specifies it should be used for exception handlers, which is what we do here.
https://docs.python.org/2/howto/logging.html#loggers