Fix missing objects for imported Github repo when github api rate limit is reached

What does this MR do and why?

Fix for issue

It turns out, that workers were not rescheduled in ReschedulingMethods because RateLimitError inherited from StandartError which was rescued before in ObjectImporter

Screenshots or screen recordings

Custom test run results importing rspec-core repo

On local GDK running multiple projects to hit rate limit:

Repo was imported with no missing objects

How to set up and validate locally

It's a bit hard to reproduce issues locally on GDK due to its slowness.

1. From the UI or API trigger import of huge repo (or a couple of repos) to reach the rate limit.
2. Check importer.log for Gitlab::GithubImport::RateLimitError
3. Check import details, all objects should be imported.