Improve error about close delimiter

I wrote code like this for servo:

``` rust
pub fn trace_option(tracer: *mut JSTracer, description: &str, option: Option<Reflector>) {
    option.map(|some| trace_reflector(tracer, description, some.reflector());
}
```

which was missing the closing ) for the map. The error however was about something different:
`error: incorrect close delimiter: `}``
