Unreachable expression warning in tail throw

Thanks for this crate! Minor annoyance I stumbled upon:

Given a function:

```rust
#[throws(())]
fn always_throw() {
    throw!(());
}
```

A compile warning pops out:

```
   | #[throws(())]
   | ^^^^^^^^^^^^^
   | |
   | unreachable expression
   | any code following this expression is unreachable

```