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
4 jobs for wip/jfelder/mpris-gsconnect-debug in 7 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #331139
flake8

00:01:31

 
  Build Flatpak
passed #331140
flatpak

00:05:59

 
  Deploy
passed #331141
review

00:00:27

manual #331142
allowed to fail manual
stop_review