Allow MergeRequestMergeabilityCheckWorker to fail

In #336979 (closed) we identified a number of issues with the MergeRequestMergeabilityCheckWorker. Here was one of the findings:

The MergeRequestMergeabilityCheckWorker will succeed (and log the error) even when the MergeabilityCheckService service check fails.

Is there a reason this sidekiq job doesn't fail instead. Setting retries to 0 and failing would make this easier to understand in production.

We should investigate the configuration of this sidekiq job to see if we could have it fail on error