surf::Exception does not implement std::error::Error

This is an unfriendly choice for errors returned by a library because it makes them not work with `?`.

```rust
async fn repro() -> Result<(), failure::Error> {
let _ = surf::get("https://www.rust-lang.org").await?; // doesn't work
Ok(())
}
```

```rust
async fn repro() -> anyhow::Result<()> {
let _ = surf::get("https://www.rust-lang.org").await?; // doesn't work
Ok(())
}
```

Application-focused error types like failure::Error are built on `impl From` which is why it matters that library errors implement std::error::Error.