Static notebook render failures end in a CalledProcessError traceback and "Internal error" #897

🤖 When nbconvert fails while rendering a static notebook (a cell raises, or the notebook JSON is malformed), rsconnect ends with a raw CalledProcessError traceback and Internal error.

Repro

cat > nb.ipynb <<'NB'
{"cells":[{"cell_type":"code","execution_count":null,"metadata":{},"outputs":[],"source":["raise ValueError('boom')"]}],
 "metadata":{"kernelspec":{"name":"python3","display_name":"Python 3","language":"python"}},"nbformat":4,"nbformat_minor":5}
NB
rsconnect deploy notebook --static -g -s $SERVER -k $API_KEY nb.ipynb

subprocess.CalledProcessError: Command '[..., '-m', 'nbconvert', '--execute', '--stdout', '--log-level=ERROR', '--to=html', 'nb.ipynb']' returned non-zero exit status 1.
Internal error: Command '[...]' returned non-zero exit status 1.

#892 prints nbconvert's stderr before re-raising and handles the "nbconvert not installed" case, but other nbconvert failures still end in a traceback.

Expected

An RSConnectException such as Rendering nb.ipynb with nbconvert failed (see output above), with no Python traceback.

Found during live QA of #892 against Posit Connect. Reproduced on 1.32.0 and current main (0287aca3).