re-expand bundle if caches directory has been cleared

Yeah. Though I was trying to be a good multithreaded citizen here:

> **Note:** Attempting to predicate behavior based on the current state of the file system or a particular file on the file system is not recommended. Doing so can cause odd behavior or race conditions. It's far better to attempt an operation (such as loading a file or creating a directory), check for errors, and handle those errors gracefully than it is to try to figure out ahead of time whether the operation will succeed.

[NSFileManager Class Reference]

With rather more verbosity, we could instead borrow [some code from `- passesSelfTest:`](https://github.com/paypal/baler/blob/master/baler/templates/BalerBundle.m.j2#L255-264) and check for the existence of the first `pathSHA1` in the bundle.