audio/transport: Propagate errors from avrcp_set_volume to DBus

audio/transport: Propagate errors from avrcp_set_volume to DBus

Any error while setting absolute volume on the peer, or notifying the
peer of changes was previously going completely unnoticed.  Propagate it
to the logs and back to the DBus "Volume" property setter so that they
aren't misled into thinking that the AVRCP command succeeded.

Note that an error is mostly harmless when the setter of the property is
an audio sink and the peer the audio source: in this case we're only
_notifying_ the peer of the change when it has already been applied on
the sink.

Also improve the other two error messages to more closely describe what
was "invalid" about the argument.