Check for null node response before cacheing

[TarzanInAJar]

init() does not handle the case where the Etcd object's list() command results in an error (maybe due to an empty configuration). A NullPointerException is thrown when cacheConfigValues tries to call "getNodes()" on a null pointer.

I made a change to init() that reports that an error has occurred/the response code if it happens, and doesn't attempt to cache anything. I also made a change to cacheConfigValues that checks for null before calling getNodes() (which shouldn't happen now, but good practice to check anyway)