remove usage of `Error::description()`

`description()` is deprecated. At the very least, it is still used in `tokio-executor/src/enter.rs:25`.

This causes unhelpful error messages on panic, like:

```
thread 'read' panicked at 'called `Result::unwrap()` on an `Err` value: EnterError { reason: "description() is deprecated; use Display" }', src/libcore/result.rs:999:5
```