Fixes and cleanups to FileOpBatch

* Fixed bug where operations that failed after precheck() would
  not properly update the Status. This could cause failed ops to
  be treated as successful.
* Removed special casing for 1-sized batches, this was mostly
  for Swift and is irrelevant after it was rewritten.
* Removed unused return value and fixed the docs

bug: 60318
Change-Id: I7f12ebf711bc196313745f943070f8bdb6335964