Successful recipe runs should not discard `stderr`

[ludwigpacifici]

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Some recipes can be run and will exit gracefully. However, when some outputs are printed on stderr, they will not be display. One example found with #325. It shows on purpose a error backtrace on stderr and it is not displayed when "run" button is clicked. If the main return a non-null value, then it will be printed.

@budziq did a first issue investigation and the behavior happens in mdbook

Two solutions:

• review the example to avoid this issue
• see with mdBook how to print stderr regardless of the main return value.