[ResponseOps] improve error logging for Slack webhook

[pmuellr]

We are seeing some errors logged from the Slack webhook connector, that don't provide much information, but should.

Here's a snippet of code where most of this is taking place:

https://github.com/elastic/kibana/blob/2932b77eec59bcb923814009d2904070f401e4de/x-pack/plugins/stack_connectors/server/connector_types/slack/index.ts#L158-L199

In our case, we could see from the log message that the retryResult() function was invoked, as it logged a unique message. However, that could have come from an HTTP response of 429 (but with no retry header) or >= 500.

• we should be able to tell the difference between these two cases
• we should include the HTTP status code in logged messages, as appropriate
• we should include the HTTP status text and/or the message of the error thrown by the Slack library, as appropriate

We should also look to see if there's a way to provide some indicator in the event log - it may be enough to ensure the event log message also includes the information we'll be logging.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)