don't catch zendesk exceptions in the worker

before this change, the sidekiq worker caught exceptions arising
as a result of trying to raise the Zendesk ticket, and feeding them
into the exception notifier. This isn't needed,
because
1) sidekiq automatically integrates with exception notifier already
2) catching the exception actually means that the retry mechanism isn't
triggered
Merge pull request #91 from alphagov/dont-catch-zendesk-exceptions-manually

don't catch zendesk exceptions in the worker