Add --fail-on-violation flag to exit non-zero when violations are detected

[mbarbero]

Is your feature request related to a problem? Please describe.
poutine always exited with code 0 after analysis, even when violations were found, making it unsuitable for use as a blocking CI gate.

Describe the solution you'd like
Add an opt-in --fail-on-violation flag that exits with code (e.g., 10) when violations are detected.

Describe alternatives you've considered
None

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I've prepared a fix in a fork. Following the CONTRIBUTING guide, if the maintainer approves this feature request, I'll create a pull request with the new feature. Thanks!

[SUSTAPLE117]

@mbarbero Hi thanks for the idea. The contribution of this new feature is very welcomed thank you!