MINOR: mux-quic: adjust error code in init failure

MINOR: mux-quic: adjust error code in init failure

If QUIC MUX cannot be initialized for any reason, the connection is shut
down with a CONNECTION_CLOSE frame. Previously, no error code was
explicitely specified, resulting in "no error" code.

Change this by always set error code in case of QUIC MUX failure. Use
the already defined QUIC MUX error code or "internal error" if unset.
Call quic_set_connection_close() on error label to register it to the
quic_conn layer.

This should help to improve error reporting in case of MUX
initialization failure.