Implement `checkResponse`

Currently `\AdamPaterson\OAuth2\Client\Provider\Slack::checkResponse` is a void method.

This should check the provider response for errors.

**See** https://api.slack.com/web

Start with AdamPaterson\OAuth2\Client\Provider\Slack::checkResponse and read the Slack Web API documentation linked in the issue to identify how provider responses report errors. Done means the previously void response check detects and handles the documented Slack error responses; verify the behavior against the provider's existing tests, if present.

索引モデルが issue の本文から書いたものです。