Slack Plugin Error Handling

As a Graylog engineer, I want to improve error handling in the Slack Notification plugin in order to better detect, report, and recover from errors.

# Input Criteria
- [x] Slack notifications plugin works

# Output Criteria
- [ ] `SlackClient` no longer uses EventNotificationExceptions
- [ ] `SlackClient` appropriately signals temporary/recoverable exceptions such as HTTP 5xx codes and network timeout errors
- [ ] `SlackClient` appropriately signals permanent/unrecoverable exceptions such as HTTP 4xx codes
- [ ] `SlackEventNotification` properly handles both temporary and permanent exceptions thrown by `SlackClient` by throwing temporary and permanent EventNotificationExceptions
- [ ] `SlackEventNotification` raises a UI notification when an permanent exception is encountered
- [ ] JUnit tests exist to validate behavior of `SlackClient`
- [ ] JUnit tests exist to validate behavior of `SlackEventNotification`

# Tasks
- [ ] Update error handling in `SlackClient`
- [ ] Update JUnit tests for `SlackClient`
- [ ] Update error handling in `SlackEventNotification`
- [ ] Update JUnit tests for `SlackEventNotification`
- [ ] Manually verify behavior