Check cursor_open return value

Summary:
This will only fail if something else has already gone wrong, but I was
hitting a crash in the baloo runner on the first cursor fetch.

We may as well catch it and handle it gracefully with the right error message.

Test Plan: Unit tests

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D25775