Do not retry updating a message if Slack returns `cant_update_message`
API [error](https://api.slack.com/methods/chat.update#errors) (meaning
bot user has no permission to update the message).
# What this PR does
- Rename `SlackClientWithErrorHandling` to just `SlackClient`
- Add more error classes + improve the way errors are raised based on
the Slack error code
- Add API call retries on Slack server errors (e.g. when Slack returns
`5xx` errors)
- Refactor some methods working with Slack API + add tests
## Which issue(s) this PR fixes
- https://github.com/grafana/oncall-private/issues/1837
- https://github.com/grafana/oncall-private/issues/1840
- https://github.com/grafana/oncall-private/issues/1842
## Checklist
- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)
