Use return codes from poetry_venv to disabiguate causes of returns.

[chris-lingwood-korelabs-co]

The issue is that setup_virtualenv in exec-env can't tell why poetry_venv has exited. There are three reasons to return:

1. if pyproject is not set, venv installation should not happen
2. else if the file doesn't exist venv installation should not happen
3. else if poetry can't show a venv path (it doesn't exist) venv installation should happen if MISE_POETRY_AUTO_INSTALL is set

• I've set cases 1 and 2 to return 1 for detection and graceful exit.
• Case 3 attempts to install if MISE_POETRY_AUTO_INSTALL is set.

The handling of cases 1 and 2 are fixed here to fix #21

[simonpercivall]

LGTM

I'm not getting an error message without setting the pyproject.toml path in mise.toml, and I do get it when it is set.