Can we simplify the above with ...

```
if (![event[kReporter_EndOCUnit_MessageKey] isEqual:[NSNull null]]) {
  [self.reportWriter printLine:@"%@", event[kReporter_EndOCUnit_MessageKey]];
}
```

This way, we can drop the WarningKey stuff.  Also, the event is now responsible for including the "failed: " prefix.  As far as the reporter is concerned, it's just an opaque message.