Report errors in workers to errbit

Notify errbit only after sidekiq has given up retrying the message
(though sidekiq is currently configured not to retry).  Airbrake is
given an exception containing details of the failure; it would be nicer
to give it the original exception so that traceback, etc, was
meaningful, but I can't see a way to do that easily and this is better
than nothing.
Remove exception mailer

This is no longer needed now that we use errbit instead.
Avoid calling .blank? on nil

Previously, nil was monkeypatched to add a .blank? method (because
`lib/ses_mailer` required `aws/ses` which required `mail` which required
`mail/core_extensions/nil.rb`, which monkeypatched nil).  We've now
removed the mailer stuff, so there are a couple of places where .blank?
was being called on nil that needed fixing.
Remove unused aws-ses gem and dependencies
Merge pull request #243 from alphagov/remove_exception_notifier

Remove exception notifier