Follow-up from "Add the PushFailureCategoryForJobs processor (sandbox-only)"

The following discussion from !3436 (merged) should be addressed:

• @hfyngvason started a discussion:

  According to Sentry, this should be a string, see https://new-sentry.gitlab.net/organizations/gitlab/issues/1534265/?project=3&query=is%3Aunresolved&referrer=issue-stream&statsPeriod=14d&stream_index=0 (internal link)