Error messages still mention "impure"

``` .rs
unsafe fn f() {
}

fn g(h: extern "Rust" fn()) {
}

fn main() {
    g(f);
}
```

produces

> foo.rs:8:6: 8:7 error: mismatched types: expected `extern "Rust" fn()` but found `extern "Rust" unsafe fn()` (**expected impure fn but found unsafe fn**)

(emphasis mine)
