Add support for new Slither fail_* options

[elopez]

Slither has added a breaking change affecting the exit code, it will now only exit non-zero in case of a traceback/error. This means that actions running after a new slither version is released will start to pass all the time unless slither crashes.

We should document this change and offer a way to adjust this behavior.

A way forward could be a new fail-on: [all/none/low/medium/high] with a default of all to maintain the current action behavior.

https://github.com/crytic/slither/blob/475de74d9b9c0329fe2f78bcc10aa60485997cc9/slither/__main__.py#L398-L424