Feature Request: fail when a rule at level "warn" doesn't trigger.

In a large installation with sub-projects, sometimes some projects break different rules than others. If the goal is to eliminate all known issues in a codebase, some sub-projects may set a rule to `warn` while the issues are worked out.

```
allProjects:
error("ReturnsNullCollection")

legacyProject:
warn("ReturnsNullCollection")
```

It would be nice for maintainers to be able to validate that all rules specified at `warn` are actually still needed at that level.

```
legacyProject:
// must be removed when last instance of this rule is fixed in legacyProject.
mustWarn("ReturnsNullCollection")
```