filebackend: improve internal use of FileBackend constants

filebackend: improve internal use of FileBackend constants

Add more result constants and split up FileBackend::UNKNOWN for
clarity. This follows up 5719815f3b, which added that constant.

Make internal FileBackendStore::doGet* methods distinguish I/O errors
from missing files; the return types of public FileBackend methods are
unchanged. Avoid process caching any mtime/size/sha1 values in the
case of I/O errors. Use error constants consistently for stat methods
when given invalid paths.

Also:
* Factor out FileBackendStore::processCacheAndPersistStatEntries() method
  to reduce significant code duplication.
* Consolidate duplicated isPathUsable() checks in FileOp subclasses to
  FileOp::precheck().
* Remove null process cache value check from FileBackend::getFileStat()
  as null values are never stored in the process cache to begin with.
* Reformat some oddly wrapped lines to look cleaner.

Change-Id: Id0e4b0da0bb2ed3184847b35142d587c7f3d953d