kio_ftp: improve error handling when copying to FTP fails

Summary:
Before:

Could not write to file /index.html.part
... and WARNING: finished() called after error()! Please fix the "kio_ftp" KIO slave

After:

Could not write to file /index.html.part
The server said: "550 Can't open that file: Disk quota exceeded."
... and no more warning.

Test Plan:
Copying a huge file onto an ftp folder in dolphin.
The server is running Pure-FTPd.

Reviewers: bruns, apol, kossebau

Reviewed By: bruns

Subscribers: kde-frameworks-devel, pino

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21290