MINOR: mux_quic: use separate error code for STOP_SENDING

MINOR: mux_quic: use separate error code for STOP_SENDING

Prior to this patch, a single error code was registrable at the QCS
level. This code was used both for RESET_STREAM and STOP_SENDING
emission. It was specified via qcc_reset_stream().

This patch extends the API so that now a dedicated error code is
implemented for STOP_SENDING as well. This may be necessary as both
frames can be sent in different context, with a diverging error code.

This patch is required to implement STOP_SENDING emission during shut
callback when read channel is closed.