Actually ErrorMessagePanel contains only one component - errorLabel - that is placed to the bottom of the wizard 
component with big Y (99), single height (1) and zero weight (0.0).

add(errorLabel, new GridBagConstraints(
        0, 99,                             // x, y
        99, 1,                             // width, height
        1.0, 0.0,                          // weight-x, weight-y
        GridBagConstraints.CENTER,         // anchor
        GridBagConstraints.HORIZONTAL,     // fill
        new Insets(11, 11, 11, 11),        // padding
        0, 0));                            // ??? (padx, pady)


If the user create a new panel with extension to ErrorMessagePanel and place some components on it all with zero width, 
then the design of such a panel would flaw. User should either make one component with weight-y >0 or add spacer panel 
(with big y and weight-y>1) after containerPanel. 
The right thing seems to be the addition of such a spacer panel that would move with guarantee the error label at the 
bottom of the panel above the errorLabel (e.g. with y=98, weight-y>1).