I tried this, but it turned out to require a lot of work compared to the importance, so I stop working on that for now.

My attempts:
https://codereview.chromium.org/2918273002/
https://codereview.chromium.org/2923163002/

1. File operation is not atomic. There is no guarantee that deleting the destination file after copy failure always works as intended. It's OK that sometimes the cleanup fails. But, for example, assume the destination file was already existed when Copy() was called and it failed. It would delete the destination file which might be different from the source file, so it leads to user data loss. It's hard to safely exclude these cases because error reporting is dependent on each file system.

2. For cancellation, it's relatively safe to delete the destination files. However, if copied entries contain directory, it becomes hard to handle it properly. We want to delete the specific file that was under copying and not the entire directory. fileManagerPrivate.onCopyProgress gives start and end event for individual files. But, it's hard to remove the last file because status.type === 'begin_copy_entry' does not contain status.destinationUrl even though 'end_copy_entry' does, and recursive directory traversal is file system dependent. (so we cannot safely predict destinationUrl from sourceUrl.)

https://codesearch.chromium.org/chromium/src/ui/file_manager/file_manager/background/js/file_operation_util.js?l=377&rcl=f5fb415739fdd30d61a3507ccd624553c8f46eef

(Thank you kinuko@ and fukino@ for help)

The problem arises because detail of directory traversal depends on each file system, and FileManager does not receive a complete list of copied files and their destinations beforehand. I first thought it does not traverse directories before copy at all, but it is also not accurate. In fact it does so on JavaScript side (fileOperationUtil.resolveRecursively_). But the retrieved information is only used for calculating progress, and there's no guarantee that the traversal performed by JS side and the corresponding file system implementation is same. If they are different, FileManager just ignores the file from copy progress.
https://codesearch.chromium.org/chromium/src/ui/file_manager/file_manager/background/js/file_operation_util.js?l=794&rcl=e5ef406c5191bf506cb24a1c6306b135ea2732fa

Related issue:  https://crbug.com/491044