There is a bug in versions of the JRE prior to 1.6 with reading indexed-color PNG files using ImageIO.read. Added special trapping of the exception for a more informative error message.

Review by: scottb (desk check)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1339 8db76d5a-ed1c-0410-87a9-c151d255dfc7