mobile: Thumbnails: improve placement of searchField and label

Summary: Move the error label to the results grid view, which fixes the layout of the TextField.

Test Plan: TextField and label are still visible if no search result is found.

Reviewers: #okular, ngraham

Reviewed By: ngraham

Subscribers: ngraham, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21551