Clean up error handling/reporting in file_sync_service.cpp.

In an earlier code review it was pointed out that there was something
very weird about fail_errno. It didn't seem to make sense that we'd
often try to continue after reporting failure. This patch cleans up
all that and assumes that if we've reported failure to the client,
we should stop what we're doing.

Bug: http://b/23437039
Change-Id: I39c38650ed9f9d5e30adbf68a7545c9e4a6ab812