core: consistent error handling semantics

define error handling semantics for the C API. The interesting aspect here is that the API is mostly asynchronous, and most errors happen asynchronously from the perspective of the caller. Additionally some errors don't have a callback to fire. For instance calling any of the API functions with a bogus stream_id means that there is no stream with callbacks to fire.

We need to determine how to surface errors in these cases and do so consistently across the API.