Handle GitHub errors

[jhaff]

Moved from #412

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Description

Fixes #409

This PR consolidates error handling logic for the case when a user's account is suspended(Octokit::AccountSuspended) and when a user's account is missing from Github (GithubPullRequestService::UserNotFoundOnGithubError). This sits in a new service object which gets passed a user and a block, GithubErrorHandler.

For both of these errors, we handle by deactivating the user, putting them in the inactive state (a new state on the state machine specifically for these users no longer accounted for in GitHub) and deleting their associated UserStat, if they have one.

Test process

• Test that a user causing this error gets deactivated and out in the inactive state
• Test that that user's UserStat is deleted, if they had one
• Test that if an unhandled error is passed to the GithubErrorHandler, it raises the error

Requirements to merge

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes