Improve visibility of error from expand-composer.php script

Problem/Motivation

If a job fails in the "expand_composer_json.php" part of the script, we capture the error and fail, like this:

- php expand_composer_json.php || EXPAND_COMPOSER_EXIT_CODE=$?
- if [[ "$EXPAND_COMPOSER_EXIT_CODE" != "" ]]; then echo "EXPAND_COMPOSER_EXIT_CODE=$EXPAND_COMPOSER_EXIT_CODE"; exit $EXPAND_COMPOSER_EXIT_CODE; fi

This is 100% correct, but when seeing the log output, it takes a bit to know that there was an error there.

Steps to reproduce

See this https://git.drupalcode.org/project/test_hook_order_parameters/-/jobs/524...

Proposed resolution

I suggest adding $DIVIDER to the error output to quickly see where the error happened.