Plugin provides non-descript error if channel not found.

### Your checklist for this issue

- [x] Jenkins version
2.204.2

- [x] Plugin version
2.37

- [ ] OS
Amazon Linux

### Description

I spent far too long trying to figure out what was wrong with my connection. When I pressed "Test Connection" I got response "failure" in read. Logs showed.

2020-04-02 22:25:35.868+0000 [id=14] WARNING j.p.slack.StandardSlackService#publish: Slack post may have failed. Response: null
2020-04-02 22:25:35.869+0000 [id=14] WARNING j.p.slack.StandardSlackService#publish: Response Code: 400

My issue was a typo in the channel name. It would be helpful if the plugin could report "channel not found" instead of "failure"

Start with the Test Connection flow and StandardSlackService#publish, using the reported 400 response and generic "failure" message as the case to trace. Confirm that a missing channel produces an explicit "channel not found" message instead, and add or update coverage for that connection failure if the existing tests cover it.

Written by the indexing model from the issue text.