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…)