CI: gate the first jb build so notebook errors cannot pass green

.github/workflows/ci.yml

@@ -55,6 +55,18 @@ jobs:
       - name: Build Download Notebooks (sphinx-tojupyter)
         shell: bash -l {0}
         run: |
+          # This is the FIRST `jb build` in the workflow, and `execute_notebooks: "cache"`
+          # means only the first build actually executes notebooks — the later HTML build
+          # reads the cache. So this is the step where a CellExecutionError surfaces, and
+          # therefore the step that has to gate.
+          #
+          # `set -eo pipefail` is required as well as `-W`: `shell: bash -l {0}` is a
+          # custom shell spec, so GitHub does not inject `-eo pipefail` (it only does
+          # that for the bare `shell: bash` shorthand). Without it a failing `jb build`
+          # would be masked by the trailing mkdir/cp, whose exit code becomes the
+          # step's — and `--keep-going` guarantees the .ipynb files exist for `cp`
+          # to succeed on.
+          set -eo pipefail
           jb build lectures --path-output ./ --builder=custom --custom-builder=jupyter -n -W --keep-going
           mkdir -p _build/html/_notebooks
           cp -u _build/jupyter/*.ipynb _build/html/_notebooks