Add TODO KF6 note

Summary:
Since there are fancy "in view" messages which show all loading trouble,
is there no need to fetch and show some error message.

See also https://phabricator.kde.org/D18117

Reviewers: KTextEditor, cullmann, dhaumann

Reviewed By: KTextEditor, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel, KTextEditor

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18116