Ignore 400 errors during html proofing

Author: vincenzobaz

.github/workflows/build.yml

@@ -19,5 +19,5 @@ jobs:
       - name: Jekyll build
         run: bundle exec jekyll build
       - name: HTMLProofer
-        run: bundle exec htmlproofer ./_site/ --only-4xx --http-status-ignore "401,429" --empty-alt-ignore --allow-hash-href --url-ignore '/https://github.com/scala/docs.scala-lang/blob/master/.*/,/www.oracle.com/'
+        run: bundle exec htmlproofer ./_site/ --only-4xx --http-status-ignore "400,401,429" --empty-alt-ignore --allow-hash-href --url-ignore '/https://github.com/scala/docs.scala-lang/blob/master/.*/,/www.oracle.com/'