Improve logging for Job retries

Currently a JobConsumer returning JobResult.FAILED is usually retried (https://sling.apache.org/documentation/bundles/apache-sling-eventing-and-job-handling.html). Unfortunately this is not exposed in the logs, so in case a JobConsumer logs an error before returning JobResult.FAILED the job is silently retried without any particular log message. I think each failed job should lead to a WARN in the log (in case the job is retried) with the attempt number and outstanding attempts in the log message. In case a job is not retried, an ERROR should be emitted to the log for every failed job.