Do not ignore error when inserting rows in 'Duplicate record'

That avoids overwriting existing record as reported in issue #1255.

This doesn't improve the underlying situation, that is inserting empty rows
before duplicating the content. But it is safer to not ignore the error in
the initial row insertion.

src/MainWindow.h

@@ -191,6 +191,7 @@ private:
     void enableEditing(bool enable_edit, bool enable_insertdelete);
     void loadExtensionsFromSettings();
     void saveAsView(QString query);
+    void duplicateRecord(int currentRow);
 
     sqlb::ObjectIdentifier currentlyBrowsedTableName() const;