Few improvements were made to how we are handling errors
while doing parallel upload/download for single object:
* unhide error under 'context canceled' which was shown in most of
cases
* add part number to error message
* don't try to commit if any error occurs while operation
* combine errors into more readable form, example:
---
failed to download part 3: uplink: eestream: failed to download stripe 0:
error retrieving piece 00: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 97.119.158.36:28967: i/o timeout
...
error retrieving piece 89: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 161.129.152.194:28967: i/o timeout
failed to download part 1: uplink: eestream: failed to download stripe 0:
error retrieving piece 01: io: read/write on closed pipe
...
error retrieving piece 97: io: read/write on closed pipe
failed to download part 2: uplink: eestream: failed to download stripe 0:
error retrieving piece 00: io: read/write on closed pipe
...
error retrieving piece 01: ecclient: piecestore: rpc: tcp connector failed: rpc: dial tcp 180.183.132.234:28967: operation was canceled
error retrieving piece 96: io: read/write on closed pipe
main.(*cmdCp).parallelCopy:418
main.(*cmdCp).copyFile:262
main.(*cmdCp).Execute:156
main.(*external).Wrap:123
github.com/zeebo/clingy.(*Environment).dispatchDesc:126
github.com/zeebo/clingy.(*Environment).dispatch:53
github.com/zeebo/clingy.Environment.Run:34
main.main:26
runtime.main:250
---
Change-Id: I9bb70b3f754567761fa8d17bef8ef59b0709e33b
4f2fae4f28