Avoid warnings from file_exists with non-existant classes. Do not cache ...

...not-found class names.

I had a case pop up in production where somehow (probably a CMS block) a class that doesn't exist was attempted to get loaded at one point. This generates warning whenever the cache is revalidated so this patch resolves it by suppressing file_exists warnings and also not caching entries that are not found (so they don't keep getting revalidated indefinitely).