This is strictly better than the previous version, approved.

Note that this method is easy to confuse with how warnings are actually converted, which is handled in LogBuildEvent, not here. Modifying this method only modifies how tasks understand that a warning they logged would be converted to an error.

Issues arise from a task relying on Log.HasLoggedError under this case:

1. All warnings are errors (warningsAsErrorsExcludingMessages will be a non-null empty set)
2. There are any warningsAsMessages for this particular project.

Result: This method returns null, which implies that there are no warnings as errors. No task will see that a warning it logged was treated as an error. While this isn't great, it's the same functionality as what came before it. The workaround for this would be to manually select warnings to be errors. I'll file a bug on it.