When a scenario hook raises an exception it prevents the scenarios from being run, but will exit with an exit status off 0.
Do you have a snippet that reproduces this that you can share so i can investigate further?
I can't seem to reproduce this using the sources in the repo.
The pip version may be behind the github repo.
Until we upgrade the pip you could install it via:
pip install git+https://github.com/gabrielfalcao/lettuce.git
@pvanderlinden can you check the scenario provided and see whether this still happens for you?