update error message outputs and handling

[garretpatten]

Pull request

Changes

• src/action.ts
  • Added buildFailureMessage() and logUnownedFilesForJob() so failures stay consistent and each unowned path is logged under a group titled Files without an effective CODEOWNERS owner (easy to search in the Actions UI).
  • Sets unownedFiles (newline-separated paths) and errorMessage before core.setFailed() so outputs are recorded reliably for later steps.
• action.yml
  • Documented the new unownedFiles output.
• .github/workflows/codeowner-verifier.yaml
  • Builds the comment from unownedFiles first (markdown bullets), then errorMessage, then a short fallback that names the log group to search.
  • Wraps the whole alert in blockquoteMarkdown() so every line is a proper > … blockquote (including GitHub’s [!WARNING] syntax).

Checklist

• Any related issues are assigned to the PR author.
• All related issues are linked from the PR.
• Changes have been reviewed and tested locally (npm test, and npm run build when src/ or action metadata changed).
• If src/action.ts or dependencies changed, dist/index.js is updated and included in the PR.