retry slack notifications incase of timeout errors

[bsushmith]

Summary
Sometimes, guardian receives a timeout from slack when trying to send a message. These messages should be retried automatically after a few seconds.

msg=\"failed to send notifications\" error=\"[appeal_id=xxxxxxxxxxx] | error sending message to user:xxxxxxxxxxxxxxx | Post \\\"https://slack.com/api/chat.postMessage\\\": context deadline exceeded (Client.Timeout exceeded while awaiting headers)\".