core: Return proper error on invalid instance id
core: Return proper error on invalid instance id

Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,jku/rygel