DefaultCheckstyleExecutor should be a failure, not an error

### Affected version

HEAD

### Bug description

Resolve this todo in DefaultCheckstyleExecutor

```
if (request.isFailsOnError()) {
// TODO: should be a failure, not an error. Report is not meant to
// throw an exception here (so site would
// work regardless of config), but should record this information
throw new CheckstyleExecutorException(message.toString());
}
```

Locate DefaultCheckstyleExecutor and start at the TODO in the failsOnError branch. Trace how the report handles this condition and inspect nearby tests or test coverage for the executor. Done means the condition is recorded as a failure rather than raised as an error, while preserving the site behavior described in the issue.

Written by the indexing model from the issue text.