mpris: Handle clients which request the volume property
Volume property handling has been recently removed from Music (commit 2bd7fb6b) but some MPRIS clients (for example GSConnect) try to get this property and this results in a crash at startup. The client should detect that the server (GNOME Music) does not handle the volume property but that's not always possible.
Apply a workaround to prevent a crash: return a DBus error.
Closes: #287 (closed)