Enhance the "retry" capabilities of JBI when there are external service errors

[bsieber-mozilla]

What happens here if for some reason, jira_client.issue_add_comment fails -- maybe there's some blip of a 500 error on Jira's end or something. Would we be in a state where only some of the comments we want on the Jira ticket appear? Should we have some sort of retry mechanism?

Originally posted by @grahamalama in #31 (comment)