fix: avoid calling handleFailed(undefined)

Created by: papandreou

We've seen this error show up in production a couple of times:

TypeError: Cannot read property 'message' of undefined
  File "/app/node_modules/bull/lib/job.js", line 273, in Job.moveToFailed
      this.failedReason = err.message;
  File "/app/node_modules/bull/lib/queue.js", line 1084, in handleFailed
        return job.moveToFailed(err).then(jobData => {
  File "<anonymous>", line unknown, in runMicrotasks
  File "internal/process/task_queues.js", line 93, in processTicksAndRejections

It seems to happen while the server instance is shutting down or after an uncaught exception, so maybe there's a method somewhere (or ioredis) that's returning a promise that gets rejected with undefined. The best I can come up with is to try to harden against it here.