Improve error messages for `assertToken`

`assertToken`, `assertEOL` have confusing error messages.
Rework globally.

Consider adding more structured error messages (markdown? fix-suggestion?).

```
print ("hello" "world")
```

error:
```
expected ")", got ""
expected end of line, found "world"
```

This should be, 
1. A single error, not two.
2. Read 
    ```
    invalid `"world"`, did you mean `)` or operator?
    ```