Improve error message for `.await` in 2015 edition

Trivial example:
```rust
fn f(x: std::pin::Pin<&mut dyn std::future::Future<Output=()>>) {
    x.await;
}
```

Output on current nightly:
```
error[E0609]: no field `await` on type `std::pin::Pin<&mut dyn std::future::Future<Output = ()>>`
 --> src/lib.rs:2:7
  |
2 |     x.await;
  |       ^^^^^ unknown field
```

Ideally the error message would suggest using the 2018 edition.