Post a PR comment if the backport runner fails (#17197)

.github/workflows/backport-assistant.yml

@@ -28,3 +28,16 @@ jobs:
           BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+)"
           BACKPORT_TARGET_TEMPLATE: "release/{{.target}}.x"
           GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
+  handle-failure:
+    needs:
+      - backport
+    if: always() && needs.backport.result == 'failure'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Comment on PR
+        run: |
+          github_message="Backport failed @${{ github.event.sender.login }}. Run: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"
+          curl -s -H "Authorization: token ${{ secrets.PR_COMMENT_TOKEN }}" \
+            -X POST \
+            -d "{ \"body\": \"${github_message}\"}" \
+            "https://api.github.com/repos/${GITHUB_REPOSITORY}/pull/${{ github.event.pull_request.number }}/comments"