AppEngine is currently at version 1.2.6 so I would expect more care given to error messages by now. Even worse though is that I am using the mature JDO framework that Google have modified to work with AppEngine. Surely after all this time they would have decent error messages being returned. These type of things really annoy me, because they waste hours of my time that could be better spent working on my project. By the time that I figure out the cause of the error I am too angry to continue on with the coding.
Surely if you are going to take the time to write a framework for others to use, especially if this is for use on your own servers for your customers to use, you would want to make the development experience as straight forward and annoyance free as possible. I am really starting to understand why Java has such a bad rap.