Use Structured Logging for MergeRequestMergeabilityCheckWorker

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

The MergeRequestMergeabilityCheckWorker uses unstructured logging which is almost impossible to investigate at the volumes we see on GitLab.com.

Switching to structured logging, or passing the error back via Sidekiq would be preferable.

See: https://docs.gitlab.com/ee/development/logging.html#gitlab-developers-guide-to-logging