Invalid char literal error message has an unhelpful snippet

`foo.rs`:

``` rust
fn main() {
    let _ = 'ab';
}
```

```
foo.rs:2:16: 2:18 error: character literal may only contain one codepoint: ';
foo.rs:2     let _ = 'ab';
                        ^~
```

The snippet at the end of the error message is not helpful, and should probably encompass everything between the two `'`s.
