MINOR: stream-int/conn-stream: Report error to the CS instead of the SI

MINOR: stream-int/conn-stream: Report error to the CS instead of the SI

SI_FL_ERR is removed and replaced by CS_FL_ERROR. It is a transient patch
because the idea is to rely on the endpoint to handle errors at this
level. But if for any reason it is not possible, the stream-interface flags
will still be replaced.