Support backward compatibility when introduce new failure reason

What does this MR do?

When we introduce a new failure reason in CommitStatus.failure_reason enum, users will no longer be able to upgrade GitLab-runner version without upgrade GitLab-rails to the latest version. Because old versions of GitLab-rails do not know the newly introduced failure reason. Simply put, we lose backward compatibility.

In this MR, we'll support the backward compatibility by setting :unknown_failure when GitLab-rails doesn't recognize the failure reason. So that users can upgrade GitLab-runner freely.

What are the relevant issue numbers?

• gitlab-runner!1051 (merged)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22224

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has ~"Requires e2 tests" label. See the Test Planning Process.