Fix forwarding failed jobs to failure queue.

This was failing because, in the new version of Sidekiq, we handle
repeatedly failing jobs with a block, rather than a method; this was
failing because it was trying to access a `logger` method on the worker
class instead of on the instance.
Fix failed job worker when no mailer is defined.

The `mailer` method on settings is only created when you set a setting
for it, so if the initialiser isn't triggered it'll fail to respond to
`settings.mailer`.
Merge pull request #143 from alphagov/fix-failure-mailure

Fix exception mailing when a job fails repeatedly