Use glass panel isntead of alert() when displaying dev mode connection failures,
because modal dialog boxes are annoying and consume unnecessary keystrokes or
mouse clicks before you are able to refresh the page.

Review at http://gwt-code-reviews.appspot.com/1449802

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10194 8db76d5a-ed1c-0410-87a9-c151d255dfc7