pr_overlap.py let a failed git or gh call escape as a bare
CalledProcessError traceback that names the command but drops the
error text, so a mistyped --base such as upstream/main (refs/heads/upstream/main
does not exist) read as an unexplained crash. The script now exits with
"pr_overlap: <command> failed with exit code N: <stderr>".
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
