refactor(api): consistent VALIDATE messages #22262

Problem:
Validation messages are not consistently formatted.
- Parameter names sometimes are NOT quoted.
- Descriptive names (non-parameters) sometimes ARE quoted.

Solution:
Always quote the `name` value passed to a VALIDATE macro _unless_ the
value has whitespace.

src/nvim/highlight.c

@@ -1089,7 +1089,7 @@ int object_to_color(Object val, char *key, bool rgb, Error *err)
     });
     return color;
   } else {
-    VALIDATE(false, "Invalid %s: expected String or Integer", key, {
+    VALIDATE_EXP(false, key, "String or Integer", NULL, {
       return 0;
     });
   }