KIO FTP: Fix file copy hanging when copying to existing file

Summary: BUG: 409954

Test Plan:
Before patch:
1. kioclient5 cp f30.png ftp://127.0.0.1/Public/f30.png
2. kioclient5 cp f30.png ftp://127.0.0.1/Public/f30.png
Kioclient will hang with debug message: `kf5.kio.core: copy() did not call finished() or error()! Please fix the "kio_ftp" KIO slave`

After patch:
1. kioclient5 cp f30.png ftp://127.0.0.1/Public/f30.png
2. kioclient5 cp f30.png ftp://127.0.0.1/Public/f30.png
All is OK, kioclient5 will exit with error status and say that file already exists.

Reviewers: sitter, dfaure, cfeck

Reviewed By: sitter

Subscribers: ngraham, rikmills, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22528