CI: gate the first jb build so notebook errors cannot pass green (#588)

The sphinx-tojupyter step is the first `jb build` in this workflow, and
`execute_notebooks: "cache"` means only the first build executes notebooks
— the later HTML build reads the cache. So this step is where a
CellExecutionError surfaces, and it could not fail.

It already carried `-n -W`, but that was inert: the step runs three
commands under `shell: bash -l {0}`, and GitHub only injects `-eo pipefail`
for the bare `shell: bash` shorthand. An explicit custom shell spec gets
neither `-e` nor `-o pipefail`, so a failing `jb build` was followed by
`mkdir` and `cp`, and the step exited with `cp`'s status. `--keep-going`
makes that worse rather than better, since it forces Sphinx to emit output
despite the errors, guaranteeing the .ipynb files exist for `cp`.

One line fixes it. The flags were already right.

Refs QuantEcon/meta#340

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

.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