DocManager: Don't show load trouble popup messages

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

Reviewers: Kate, cullmann, dhaumann, mwolff

Reviewed By: Kate, cullmann

Subscribers: kwrite-devel, Kate

Tags: Kate

Differential Revision: https://phabricator.kde.org/D18117