Inspecific span when struct has no field

``` rust
struct Foo {
    f: uint
}

struct Bar {
    foo: uint
}

fn foo(f: Bar) -> Foo {
    Foo {
        foo: f.foo
    }
}
```

```
foo.rs:11:8: 11:18 error: structure has no field named `foo`
foo.rs:11         foo: f.foo
                  ^~~~~~~~~~
error: aborting due to previous error
```

A different error is given if f has no field `foo`.
