src/demux.c: send error message (Fixes CID #134924)
src/demux.c: send error message (Fixes CID #134924)

The function dvbpsi_DetachDemux(), which is now a wrapper around dvbpsi_chain_demux_delete()
failed to check the return value. Now it returns an error message when the return value is 'false'.

mwgoldsmith/dvbpsi,mwgoldsmith/dvbpsi,mwgoldsmith/dvbpsi,mwgoldsmith/dvbpsi