Before this change multi-thread copies using the FTP backend used to error with
551 Error reading file
This was caused by a spurious error being reported which this code silences.
Fixes#7532
See #3942
Before this fix, when a write to a read only directory failed, rclone
would leav spurious directory entries in the directory.
This confuses `rclone serve webdav` into giving this error
http: superfluous response.WriteHeader
This fixes the VFS layer to remove any directory entries where the
file creation did not succeed.
Fixes#5702
WeidiDeng