neovimUtils.grammarToPlugin: improve error message on invalid grammarPlugins (#357062)

Author: khaneliman

pkgs/applications/editors/neovim/utils.nix

@@ -176,7 +176,11 @@ let
         (lib.replaceStrings [ "-" ] [ "_" ])
       ];
 
-      nvimGrammars = lib.mapAttrsToList (name: value: value.origGrammar) vimPlugins.nvim-treesitter.grammarPlugins;
+      nvimGrammars = lib.mapAttrsToList (
+        name: value:
+        value.origGrammar
+          or (builtins.throw "additions to `pkgs.vimPlugins.nvim-treesitter.grammarPlugins` set should be passed through `pkgs.neovimUtils.grammarToPlugin` first")
+      ) vimPlugins.nvim-treesitter.grammarPlugins;
       isNvimGrammar = x: builtins.elem x nvimGrammars;
 
       toNvimTreesitterGrammar = callPackage ({ }: