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.

test/functional/api/buffer_updates_spec.lua

@@ -762,7 +762,7 @@ describe('API: buffer events:', function()
   it('returns a proper error on nonempty options dict', function()
     clear()
     local b = editoriginal(false)
-    eq("Invalid key: 'builtin'", pcall_err(buffer, 'attach', b, false, {builtin="asfd"}))
+    eq("Invalid 'opts' key: 'builtin'", pcall_err(buffer, 'attach', b, false, {builtin="asfd"}))
   end)
 
   it('nvim_buf_attach returns response after delay #8634', function()