Code review comment for lp:~phablet-team/media-hub/fix-1479036

Some issues, mostly related to handling this in the caller of these functions instead so errors can be returned in the DBus response. An example where this is already performed is in ServiceSkeleton::handle_set_current_player().

The only exception is probably the handler of player->on_client_disconnected(), as the trigger is not a DBus function in that case.