nixos/mopidy: restart the systemd service on failure

This ensures Mopidy is running, even after crashes.

nixos/modules/services/audio/mopidy.nix

@@ -83,6 +83,7 @@ in {
       description = "mopidy music player daemon";
       serviceConfig = {
         ExecStart = "${mopidyEnv}/bin/mopidy --config ${concatStringsSep ":" ([mopidyConf] ++ cfg.extraConfigFiles)}";
+        Restart = "on-failure";
         User = "mopidy";
       };
     };