Quay stops indexing after Clair failure

When an image is pushed to Quay, the worker process will send an indexing request to Clair. If Clair returns an error it will stop the worker process. If subsequent images are queued for scanning/indexing the worker process will never get around to sending those indexing requests to Clair and the erroring image will stop the worker process prematurely. 

We should continue indexing after Clair returns an error as the error is linked to the request for that manifest and not to the entire worker job.