Set dir list job cancellation error to mild
Set dir list job cancellation error to mild

Fixes https://github.com/lxqt/pcmanfm-qt/issues/789

A directory list job may be legally cancelled at `Folder::reload()`, in which case, pcmanfm-qt shouldn't show an error message (pcmanfm-qt only shows errors with severities greater than `ErrorSeverity::MILD`).

lxde/libfm-qt,lxde/libfm-qt