Remove unwanted `unwrap`s

Perhaps neither of the below needs to be `unwrap`.

```
src\token.rs
68: String::from_utf8(self.text.to_vec()).unwrap()

src\driver.rs
139: token.offset() - text[0..token.offset()].rfind('\n').unwrap()
```

See if you can turn them into real `Error`s, using `thiserror`. Add test cases.