nixos/desktopManager: Only trace if the default session isn't found

The default session might be found in `extraSessionFilePackages`, but it's not
viable to detect at evaluation time, so emit a warning.

In LightDM instead of checking `defaultSessionName` against
`displayManager.session.names` we rely on the assertions in
`desktopManager` and `windowMananger` and just check that there's at least one
default set. The second assertion could never actually be triggered.

nixos/modules/services/x11/desktop-managers/default.nix

@@ -96,13 +96,13 @@ in
           else if any (w: w.name == defaultDM) cfg.session.list then
             defaultDM
           else
-            throw ''
-              Default desktop manager (${defaultDM}) not found.
-              Probably you want to change
-                services.xserver.desktopManager.default = "${defaultDM}";
-              to one of
+            builtins.trace ''
+              Default desktop manager (${defaultDM}) not found at evaluation time.
+              These are the known valid session names:
                 ${concatMapStringsSep "\n  " (w: "services.xserver.desktopManager.default = \"${w.name}\";") cfg.session.list}
-            '';
+              It's also possible the default can be found in one of these packages:
+                ${concatMapStringsSep "\n  " (p: p.name) config.services.xserver.displayManager.extraSessionFilePackages}
+            '' defaultDM;
       };
 
     };