Added checking for session names for jovian.steam.defaultSession

This checks for desktop session names for jovian.steam.desktopSession. It tells you if the input is in the list of valid names that the system accepts, and also checks if the option is unset, in which case it will default to gamescope-wayland.

modules/steam/autostart.nix

@@ -31,7 +31,15 @@ in
         };
 
         desktopSession = mkOption {
-          type = types.nullOr types.str;
+          type = with types ; nullOr str // {         
+            check = userProvidedDesktopSession:
+              lib.assertMsg (userProvidedDesktopSession != null -> (str.check userProvidedDesktopSession && lib.elem userProvidedDesktopSession config.services.xserver.displayManager.sessionData.sessionNames)) ''
+                  Desktop session '${userProvidedDesktopSession}' not found.
+                  Valid values for 'jovian.steam.desktopSession' are:
+                    ${lib.concatStringsSep "\n  " (lib.lists.remove "gamescope-wayland" config.services.xserver.displayManager.sessionData.sessionNames)}
+                  If you don't want a desktop session to switch to, remove 'jovian.steam.desktopSession' from your config.
+              '';
+          };
           default = null;
           example = "plasma";
           description = lib.mdDoc ''