Provide better classification for renderer errors

StreamingRenderer is based on the concept that errors should be handled but they should not stop rendering.
This is good for small errors, but may make debugging a server side error (such as an srid mismatch) or an
SLD error hard to handle.

Without changing the overall design, I'd like StreamingRenderer to emit error events somehow classified
against their severity or nature (datastore, projection, sdl related and so on),
so that the app using it can decide wheter it's worth going on with the rendering process or if it's better to
stop and provide a clear error report to the user.