Don't override job state when stopped

Right now, when a job runner is stopped (as happens when the JR is shutting down + requests are being suspended), it overrides its job's return state, so the job can never return STATE_FAIL. I know the idea of overriding STATE_FAIL with STATE_STOPPED was that some of jobs probably just return failed when they should really be returning stopped, but with the overriding as is there’s no way to indicate if something really did go wrong when stopping a job and we don’t want to retry it / resume the request

Relevant code here: https://github.com/square/spincycle/blob/master/job-runner/runner/runner.go#L151-L156