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.

Hmm. I might be hypoglycemic. I think that `contentsOfDirectoryAtPath:` was indeed a bit safer.

In any case, will switch to `fileExistsAtPath:` for all of the above-discussed reasons.