{Binding,NormalizedNode}StreamEventWriter lack IO error reporting

While implementing JSON serializer, we have realized that the codec infrastructure has two major flaws:

• the API methods do not allow an implementation to report an IOException, which is common to anything Writer-based (like GSON's JsonWriter)
• the API lacks a flush() method which would ensure consistency

Given this APIs are expected to remain stable after Helium release and be widely used to interact with outside world, we need to fix them before the release.