AI: Celery workers lose their cache after an exception causing many hits to S3

Fallback to file-based cache for classifier data
Add info-level logging for cache hits/misses

[TIM-652](https://edx-wiki.atlassian.net/browse/TIM-652): Celery workers lose their cache after an exception causing many hits to S3

@stephensanchez This is now ready for review :)
Does this mean that all the classifiers will be stored on disk on the server, as well as S3? There should not be any issue given the size of the classifiers I guess