re-expand bundle if caches directory has been cleared

D'oh. Right - my use of `contentsOfDirectoryAtPath:` was certainly no safer than `fileExistsAtPath:` would have been. Merely more expensive.

Yeah, I'd be okay with simply checking for directory existence for now. That would cover the case that prompted this change (in which an app purposely cleared the caches directory [for reasons unclear to me]), as well as most (all?) low-memory cases where the OS clears the caches directory.

(Since whatever code that calls into here is probably UI-related code, and therefore running on the main thread, multithreaded concerns are actually unlikely to be a big deal, I'd think.)