Improve diagnostic for `break` in `async { ... }` block

[Centril]

With:

#![feature(async_await)]

fn foo() {
    async {
        break;
    };
}

you get:

error[E0267]: `break` inside of a closure
 --> src/lib.rs:5:9
  |
5 |         break;
  |         ^^^^^ cannot break inside of a closure

Note how it references "a closure" which seems confusing for a user who doesn't presume the implementation.

[gilescope]

@rustbot claim

[gilescope]

I'm assuming its ok to leave this as the same error code of E0267 - in essence it's the same error, just more specific text.

[Centril]

Yeah I think it's fine.