Commands in the `release` module always return exit code 0, even if they fail

Hi,
When executing commands such as nbdev_pypi from a shell (which get routed to nbdev.release:release_pypi), the commands always return exit code 0 regardless of the errors happening inside the command.

I ran into this issue while using nbdev_pypi inside my CI/CD. If I forget to bump the version, for example, the twine upload command will fail with a "409 Conflict" error, but the nbdev_pypi command still returns error code 0 (success).
This is problematic, because there is no way to catch this error and do something useful (like fail the CI/CD build).

The code for release_pypi uses os.system calls like:

system(f'twine upload --repository {repository} {_dir}/dist/*')

https://github.com/fastai/nbdev/blob/4af4d479c78880f4a18af4254b119f8af8b3a8a4/nbdev/release.py#L307-L313C68

As far as I can see, the call to os.system() returns the exit code as a 0/1 but does not throw an exception.

I'm happy to contribute a pull request to fix this, but would like some guidance on what is the right way to go about it.
Options I see:

1. Change code to use subprocess instead of os.system()
2. Keep using os.system, but instead of calling it directly - add a wrapping function like execute_throw_on_error() which will check the response code and throw exception if it's not 0.
3. Somehow handle it in a more generic way in the the call_parse decorator (but what do we do if there are multiple calls to os.system and we want to fail if either fails?)

Thanks!

Julian.

Beginne in nbdev/release.py bei release_pypi und dem zugehörigen os.system-Aufruf für twine upload. Untersuche die anderen Release-Befehle auf dieselbe Behandlung des Exit-Status und reproduziere anschließend einen fehlgeschlagenen Upload wie den gemeldeten 409 Conflict. Erledigt ist die Aufgabe, wenn ein fehlgeschlagener Befehl einen Exit-Code ungleich null weitergibt, sodass CI/CD den Fehler erkennen kann.