Caching

This adds caching for the locale JS assets and improves the diagnostic output when asset retrieval/caching goes awry.
Not sure if we need to spit out the cache path each time we're about to cache a file. Seems like once is enough, especially because we show the path to the file that was saved. I'm seeing a lot of "Cache path: ...." in my log output:

![1__ssh](https://cloud.githubusercontent.com/assets/873785/4560037/c0906d16-4ef1-11e4-9e06-da298a33f6a9.png)