S3Guard fsck: Use enum instead of int for severity in violations

Looking in the code, I can see that it's a severity level "2", so not that serious.

1. We should go away from and int value here to something comprehensible, i.e. an enum.
2. Or at least some contents used in the Violation enum setup.
3. Either way, how about letting us choose which level to fail on.
4. And maybe severity 2 should be logged at WARN, not ERROR, as an early user was confused "why an error yet none reported in status"