file ioslave: stop copying as soon as the ioslave is killed

Summary:
Previously when cancelling a copy the ioslave would continue.
The file copy job would then remove the file.
The dest file being removed would cause an IO error in the slave resulting in warnings such as :

kf5.kio.kio_file: Could not change permissions for "...
kf5.kio.kio_file: Couldn't preserve group for "...
kf5.kio.kio_file: Couldn't preserve access and modification time for "...

Preserve clean up of intermediate file copy leftover.

Test Plan:
Copy a file, cancel the copy -> the destination file is cleaned
Copy a file, overwriting another -> the .part intermediate file is cleaned

Reviewers: Frameworks, dfaure, ngraham, apol

Reviewed By: dfaure

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25117