How to generate guzzle exceptions when calling Slack API and getting errors

guzzle.php

<?php

$stack = new \GuzzleHttp\HandlerStack(\GuzzleHttp\choose_handler());
$stack->push(function (callable $handler) {
          return function (
              \Psr\Http\Message\RequestInterface $request,
              array $options
          ) use ($handler) {
                $promise = $handler($request, $options);

                return $promise->then(
                    function (\Psr\Http\Message\ResponseInterface $response) use ($request) {
                        $data = \GuzzleHttp\json_decode((string) $response->getBody(), true);

                        if (!$data['ok']) {
                            throw new \GuzzleHttp\Exception\RequestException($data['error'], $request, $response);
                        }

                        return $response;
                    }
                );
            };
        });
$client = new \GuzzleHttp\Client(['handler' => $stack]);