Inform of auto-reformatter on failed clang-format

Creates a message in the Clang Format GitHub workflow's logs which
directs the PR author to the auto-reformat trigger.

Previously, people were not aware that they could run clang-format via
writing a comment with the trigger. Hopefully, this message will be
enough.

Having the GitHub bot respond with a comment informing the PR author of
this feature was considered, but it was dropped due to technical
limitations in GitHub. A PR from a fork has a different GITHUB_TOKEN
secret as the main repo, preventing the secret from being used to add a
comment.

Author: Quincunx271

.github/workflows/clangformat.yml

@@ -18,3 +18,14 @@ jobs:
       with:
         source: '.'
         clangFormatVersion: 9
+
+    - name: Report failure
+      if: ${{ failure() }}
+      run: |
+        cat << EOF
+        ::error::Something is not properly formatted by clang-format.
+
+        ::error::If you add a comment to your PR with exactly the text 'Do: Reformat' \
+        (without the quotes), each commit of your PR will be reformatted for you.
+        EOF
+        exit 1