MainWindow: Don't show error message when cancelling extenstion loading

Don't show a file-not-found message box when cancelling the open file
dialog to select a SQLite extension.
Don't call populateTable() twice on startup when it's not needed

We need to call MainWindow::populateTable() when an SQL script was
executed during startup to make sure the latest data is shown. There is
however no need to do so when no SQL script was specified.