Improve rate limit missing

[deathaxe]

This PR builds on recent changes made by @wbond to fix some rate-limit related issues and backports changes from four-point-oh branch targetting same sort of issues.

It also adds RateLimitSkipException exception, which is raised if a url is crawled after rate limit was hit. It may therefore also be associated with failed sources, packages or libraries.

Note packages and dependencies can be assigned those exceptions when rate limit is hit when downloading release information by following lines (packages):

https://github.com/wbond/packagecontrol.io/blob/9f5eb7e3392e6bc2ad979ad32d3dd27ef9c00b20/app/lib/package_control/providers/repository_provider.py#L818-L819