Don't retry if Navvy::Job::NoRetryException is raised by the job

[rlivsey]

Most of the time if a job fails it's fine to retry later, however some things aren't recoverable and there's no point constantly retrying. For these things I still want to fail the job so that it comes up in our admin interface as failed and I can investigate it, but I don't want it to retry.

To this end, I added an exception (Navvy::Job::NoRetryException) which if raised by the job means that it won't bother queuing it up for a retry.