Better error message for illegal yield in classic mode

```js
async function g() { // note: not a generator function
    yield 42;        // same error message without the semicolon
}
```
Raises `unexpected 'yield' keyword` in ESM mode but `expecting ';'` in classic mode.

It's not strictly wrong because `yield` is a valid identifier in classic mode (you get the same error with `yolo 42`) but it's unhelpful.