Catch all failures from repomd loads, add warnings, docs

There are roughly eleven quintillion failure modes when loading repomd, with zero of them being actionable in any realistic way. Swallow all failures (not just HTTPErrors) and send back NoneTypes from the subprocess, the callers will similarly roll those over into "no package for you" results.
Since the YumFinder is designed with the full expectation that a viable, persistent cache is in-place for it to do its work, add an explicit warning if a YumFinder is initialized with the null cache
Add documentation on how to use caching.
launchpadlib is not thread-safe, and its objects cannot be pickled (https://bugs.launchpad.net/launchpadlib/+bug/822847) which means we cannot use dogpile.cache with it, as it assumes both.
But the call to initialize launchpadlib is expensive, so attempt to minimize remote calls within threads, at least. This will cause duplicated work in concurrent applications, which sadly cannot be avoided.

This change is

juledwar / soufi