Cherry pick beefed-up error handling in the CopyService from master. DO NOT MERGE

Send copy errors to the destination provider.
Big overhaul of tests to make them simpler.
Test that errors during copying are detected and partial copies are
cleaned up.
Test that copying empty directories works properly.

Change-Id: I3fe0e73bdc92c2b6f522857ca5631f6d03d5a666
(cherry picked from commit dae8c378d6c680ae9efa8e1202d9bc92fb0dfb11)