Handle failed requests when posting notifications to Slack/Mattermost

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Summary

With !86168 (merged) we started logging error responses from the Slack/Mattermost API when posting notifications in app/models/concerns/integrations/slack_mattermost_notifier.rb.

Some of them are rate limit responses (HTTP 429 errors) which we plan to handle in #356896, but we should investigate other errors too and decide how to handle them.

Improvements

Errors can be seen at https://log.gprd.gitlab.net/goto/76be1c00-cb77-11ec-b73f-692cc1ae8214

Some of them like team_disabled and channel_not_found could be handled by disabling the integration, and maybe notifying users somehow.