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