Fixes#7619
The writeToFile function had two critical bugs that could cause data loss
during EC shard evacuation when the destination disk is full:
Bug 1: When os.OpenFile fails (e.g., disk full), the error was silently
ignored and nil was returned. This caused the caller to think the copy
succeeded.
Bug 2: When dst.Write fails (e.g., 'no space left on device'), the error
was completely ignored because the return value was not checked.
When evacuating EC shards to a full volume server (especially on BTRFS):
1. OpenFile may succeed (creates 0-byte file inode)
2. Write fails with 'no space left on device'
3. Errors were ignored, function returned nil
4. Caller thinks copy succeeded and deletes source shard
5. Result: 0-byte shard on destination, data loss!
This fix ensures both errors are properly returned, preventing data loss.
Added unit tests to verify the fix.
fdb888729b