Make `Failure` public

[rustworthy]

Users should be able to read Failure's fields.

There is also a field in that structure not covered just yet called remaining, meaning the number of remaining retry attempts.

We will probably want to implement #61 first, so that we can "force-push" failed jobs from retries set to sheduled set, then

• in our test - consume the job again and examine it's failure field via Job::failure getter. Currently, we would have to wait for the Faktory server to do so or - alternatively - go to the Web UI and re-queue the job from there, which is - in both cases - not suitable for auto-tests.

[jonhoo]

That seems reasonable to me! Though let's make sure we set it to #[non_exhaustive].