Dict Engine: improve error handling.
Summary:
Instead of returning "<dl>\n<dt><b></b></dt>\n<dd></dl>"
when the word doesn't exist, return a proper readable string so users
know that the lookup is done and nothing was found.
Test Plan: I'm porting the dict applet to QtQuick, review request coming soon.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: broulik, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8419