re-enable build failures on warnings

Author: mmcky

.github/workflows/ci.yml

@@ -61,7 +61,7 @@ jobs:
         shell: bash -l {0}
         run: |
           rm -r _build/.doctrees
-          jb build lectures --path-output ./
+          jb build lectures --path-output ./ -nW --keep-going
       - name: Save Build as Artifact
         uses: actions/upload-artifact@v1
         with: