Bail out on FileBackend operations if the initial stat calls failed

Bail out on FileBackend operations if the initial stat calls failed

* The preloadFileStat() call acts as a good canary for whether all
  affected servers are up. If anything failed in it, then it's not
  worth bothering to send the actual write requests.
* Also made FileBackend::preloadFileStat abstract while at it since
  the two subclasses implemented it and so should any others.
* Fixed a silly comment typo.

Change-Id: I5bd427f654aa4a9d6bfe4ed7566276e8ac520b30