wasi-http errors

errors.wit

/// `request-write-result` will produce an error if the _caller_ has trouble writing
/// the request body or trailers.
///
/// The returned `future` will produce an error if the _callee_ has trouble writing
/// the response body or trailers.
handle: func(request: request, request-write-result: future<result<_, error-code>>)
  -> result<tuple<response, future<result<_, error-code>>>, error-code>;