Make request timeouts a top-level error

[KodrAus]

Related to #266

reqwest has a specific error variant for timeouts. We could surface methods on ClientError that can be used to determine whether or not the error is due to a timeout:

let response = client.search::<MyType>().send;

match response {
    Ok(docs) => { .. },
    Err(Error::Api(ApiError::IndexNotFound { .. })) => { .. },
    Err(Error::Client(err)) if err.is_timeout() => { .. },
    _ => { .. }
}

It's introducing a bit of a different paradigm for dealing with variants on the client error, but I think this is ok. It's convenient to be able to pull properties out of API erorrs, like the index name and use that property to create the index.

[KodrAus]

Hmm, looks like reqwest currently doesn't make it easy to determine whether an error was because of a timeout (see seanmonstar/reqwest#229).

This will also depend on #290 so we can properly interrogate captured Errors and downcast them.