Fixed job column headings in overview.
Job history.
Implement web display of job failures.
Fixed error in Job integration test canCompleteItself(). canHearbeatItself() test now passes. Implemented Job::complete() and ::heartbeat() methods.
raisesAnErrorIfItFailsToHeartbeat() integration test now passes.
raisesAnErrorIfRetryFails() Job integration test now passes. Required implementing Job retry() methods.
Relocate events integration test into integration package. Got missed with others.