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
5 jobs for wip/jfelder/mpris-gsconnect-debug in 11 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #332665
flake8

00:04:08

failed #332650
flake8

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  Build Flatpak
passed #332651
flatpak

00:06:13

 
  Deploy
passed #332652
review

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passed #332653
allowed to fail manual
stop_review

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