Don't treat KJob::UserDefinedError as unknown error
KIO's jobs use UserDefinedError in several places for errors where there is no specific error code
When converting the error code to string it is treated as unknown, suggesting to report a bug to bugs.kde.org
However, the way we use UserDefinedError it's not necessarily a KDE bug, we use it for example when a desktop file's executable is not found
Instead treat it the same as ERR_SLAVE_DEFINED, i.e. show the message, but without the special text around it
Edited by Nicolas Fella