Evaluate `ok` in response of every Slack API call and handle gracefully

See this link https://api.slack.com/web

All Web API responses contain a JSON object, which will always contain a top-level boolean property ok, indicating success or failure.

For failure results, the error property will contain a short machine-readable error code. In the case of problematic calls that could still be completed successfully, ok will be true and the warning property will contain a short machine-readable warning code (or comma-separated list of them, in the case of multiple warnings).