checks on input, exceptions, improved contains query performance

gui/mainwindow.h

@@ -42,8 +42,8 @@ class MainWindow : public QMainWindow
 	void lineEditReturnPressed();
 	void handleSearchResults(const QVector<SearchResult> &results);
 	void handleCancelledSearch();
+	void handleSearchError(QString error);
 	void treeSearchItemActivated(QTreeWidgetItem *item, int i);
-
 	void showSearchResultsContextMenu(const QPoint &point);
 	void tabChanged();
 	void pdfPreviewReceived(PdfPreview preview);