repo-updater: debt: ugly rate limit handling code introduced by me in migration codepath

Created by: slimsag

In https://github.com/sourcegraph/sourcegraph/pull/3617 I introduced an ugly fix for #3590 (closed) by using string error comparison and a sleep statement.

As mentioned in the TODO, ideally the underlying codehost source ListRepos would have an option passed down to it via the migration codepath which would cause it to slow down when rate limiting is encountered. I did not do it this way because I wasn't confident in my ability to do so quickly for the emergency patch release without introducing other potential regressions.

It would be good to remove this code sooner rather than later in favor of a proper fix, but probably can be done in just 3.4 with no need for a subsequent patch release I think.