do something with message responses

[nwilging]

Currently message responses are unhandled, e.g. - https://github.com/nwilging/laravel-slack-bot/blob/develop/src/Services/SlackApiService.php#L104

In the case of a request failure an exception would be thrown, these should not be caught by the package. However slack often sends back validation related messages in valid 200 responses, so it would be useful to return something from these messaging methods that consumers can use.