When copying/evacuating empty volumes, the .idx file may not exist on disk
(this is allowed by checkIdxFile for volumes with only super block in .dat).
This fix:
1. Uses os.IsNotExist() instead of err == os.ErrNotExist for proper
wrapped error checking in CopyFile
2. Treats missing source file as success when StopOffset == 0 (empty file)
3. Allows checkCopyFiles to pass when idx file doesn't exist but
IdxFileSize == 0 (empty volume)
Fixes volumeServer.evacuate and volume.fix.replication for empty volumes.
4dcd33bbc8