Add custom merge error for unknown flake output attrs

modules/flake.nix

@@ -10,7 +10,23 @@ in
     flake = mkOption {
       type = types.submoduleWith {
         modules = [
-          { freeformType = types.lazyAttrsOf types.raw; }
+          {
+            freeformType =
+              types.lazyAttrsOf
+                (types.unique
+                  {
+                    message = ''
+                      No option has been declared for this flake output attribute, so its definitions can't be merged automatically.
+                      Possible solutions:
+                        - Load a module that defines this flake output attribute
+                          Many modules are listed at https://flake.parts
+                        - Declare an option for this flake output attribute
+                        - Make sure the output attribute is spelled correctly
+                        - Define the value only once, with a single definition in a single module
+                    '';
+                  }
+                  types.raw);
+          }
         ];
       };
       description = ''