Error logging on bad create_gh_issue return value
Handle milestone_id == 0 the same as nil

A `milestone_id` of `0` (instead of `nil`) has been seen coming out of
the Lighthouse API, for a ticket that has no milestone in the Lighthouse
web interface. A request to `.../milestones/0.json` then yields a 404,
causing the migration to error out.

So, interpret `milestone_id == 0` the same as `milestone_id.nil?`
Sleep and retry once on GitHub API 502 Bad Gateway

... in the three places where I encountered it.
Log args when create_gh_issue returns bogus value

To aid troubleshooting.
Merge pull request #7 from gthb/patch-2

More improvements prompted by hiccups in our migration