Improve error-message when rate-limit is hit.

Prior to this commit, on a HTTP error, a general (and generally useless)
error was printed, and the issue being requested was skipped.  This was
true even when requests had hit the github rate-limit.

This commit improves the github_issue_requester's reporting slightly to
detect when rate-limiting has occurred, and prompt the user to wait
before retrying.

Issue #16