CI: Enforce updating the CHANGELOG in the CI

Currently, this is only returning warnings, but we seem to just skip
them. As it's possible to merge PRs when the CI is red, issuing an error
would help us to think about populating this file.

.github/workflows/check_pr.yml

@@ -31,5 +31,6 @@ jobs:
         shell: bash
         run: |
           if git diff --exit-code "origin/${BASE_REF}" -- CHANGELOG.md; then
-              echo "::warning::No CHANGELOG.md modifications were found in this pull request."
+              echo "::error::No CHANGELOG.md modifications were found in this pull request."
+              return -1;
           fi