Private type error has poor span and inaccurate phrasing for private traits used as bounds

``` rust
pub fn foo<T: Bar>() {}

trait Bar {}
```

```
priv-trait.rs:1:12: 1:13 error: private type in exported type parameter bound
priv-trait.rs:1 pub fn foo<T: Bar>() {}
                           ^
error: aborting due to previous error
```

The span should point to `Bar` (especially important for param lists with several traits), and it could probably say `trait` instead of `type`.
