Use the sugary syntax to print the `Fn` traits in error messages

This is to encourage the use of the sugary syntax instead of the `<>` syntax, which will not be usable post-1.0. Rustdoc [still uses the `<>` syntax](https://github.com/rust-lang/rust/issues/19909), so if a rustdoc wizard is looking for something to do, it would be nice to use the parenthetical syntax there as well. (I tried to patch rustdoc as well, but failed…)
Could this logic for `is_fn_trait` be refactored into the `parameterized` function? You could just pass in the `def_id` and it could internally check for these traits.