Document edge cases of the appveyor cache

[aidanhs]

https://www.appveyor.com/docs/build-cache/

For example:

• if a cache save or cache restore fails, does the build fail? "It’s a hard quota which means the build will fail while trying to upload cache item exceeding the quota" implies that it will fail, but as of Build should not fail because of cache errors #723 I believe it doesn't, so perhaps this needs updating
• if a cache fails part way through restoring, will it make sure the cache directory is clear before continuing? Or should users validate the cache before relying on it? (similar question for failing part way through saving - will a corrupt cache potentially be restored on the next run?)

Unfortunately, the latter is not a hypothetical question - https://ci.appveyor.com/project/rust-lang/rust/build/1.0.2623/job/46o90by4ari6gege#L5