handle slack error 429

[fberrez]

Description

This modifies the behavior when an error 429 - Too Many Requests occur when it sends a request to slack. It now prints a log instead of raising an error.

Before that, the log that occured the failure was pushed in a "pending" queue. This queue doesn't wait long enough to send another request to slack, resulting in another 429.

Checklist

• [x] I have reviewed the contributing guidelines.
• [x] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [x] I have updated the documentation.
• [x] I have updated the changelog.

[nsano-rururu]

Execution of test code is failing.

[nsano-rururu]

@fberrez Please correct the test code so that it does not cause an error.

[JeffAshton]

Doesn't this now result in the alert getting lost? Like how does Elastalert know how to retry later?

[nsano-rururu]

@jertel

I think this pull request with no response should be closed.