fail black_formatter step if there are suggestions (#768)

Author: dlawin

.github/workflows/formatter.yml

@@ -21,3 +21,6 @@ jobs:
         uses: reviewdog/action-suggester@v1
         with:
           tool_name: blackfmt
+      - name: Fail if there are formatting suggestions
+        if: steps.black_formatter.outputs.is_formatted == 'true'
+        run: exit 1