Improve error message for `[***foo**]`

```
Error: Unexpected [***args**]{.parameter-name} [:]{.parameter-annotation-sep} [[TagAttrs](%60htmltools.TagAttrs%60)]{.parameter-annotation} [=]{.parameter-default-sep} [()]{.parameter-default}`</code>`{=html} at 0:17
```

This isn't very helpful.