pipewire-alsa: Propagate stream errors
Propagate stream errors and check for successful negotioation or error in _prepare(). This will return an error from _prepare() if there is no target node to link to (unless PIPEWIRE_AUTOCONNECT=false).
It would be preferable to also set PW_STREAM_FLAG_INACTIVE to not start processing until _start() is called but then it would not be possible to signal successful negotiation.