Modal dialog form validation demo - error message uses hard-coded color

[fgTodd]

On the modal form demo, there is a validation message that appears at the top of the dialog when the submit button is pressed. This seems to have a hard-coded light yellow background color instead of using a framework class so on darker themes (UI darkness, Dark Hive) where the ui-widget-content text color is white, you see a light yellow bar with white text on it and it looks broken. ​http://jqueryui.com/demos/dialog/#modal-form

The fix is to use add the ui-state-highlight or ui-state-error to this message div so the text color and background color will coordinate and be legible.

[Scott González]

Fixed in #3175.