Handle GitHub Repository Invite revoke properly if user has no OAuth account

[sentry-io]

Sentry Issue: SERVER-1F1

RuntimeError: No active exception to reraise
  File "polar/worker.py", line 294, in wrapper
    r = await f(*args, **kwargs)
  File "polar/benefit/tasks.py", line 151, in benefit_revoke
    await benefit_grant_service.revoke_benefit(
  File "polar/benefit/service/benefit_grant.py", line 242, in revoke_benefit
    properties = await benefit_service.revoke(
  File "polar/benefit/benefits/github_repository.py", line 183, in revoke
    raise