Adjust Notification error icons colors

[tomlyn]

The following info was from Devin:
https://github.ibm.com/NGP-TWC/wml-canvas-planning/issues/9521#issuecomment-28748104

The error/warning icons in the notification panel should be adjusted as follows:

1. The little white part inside the error icon should use $ui-background

2. According to https://pages.github.ibm.com/cdai-design/pal/patterns/status-icons, the "!" in the warning icon for both dark and the light themes should have a dark gray. $ui-05 should be ideal.