Trying to generate an ImageResource yields NPE, but not where it should, and
hey, look at that assert that you think won't really be called. I called it!

Add the resource name which is wicked helpful.

Review by: fabbott@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10591 8db76d5a-ed1c-0410-87a9-c151d255dfc7