Add graceful failure when a client sends a deleted EntityProxy in a request.
Resolves GWT issue 5403.
Patch by: bobv
Review by: rjrjr

Review at http://gwt-code-reviews.appspot.com/986802


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9034 8db76d5a-ed1c-0410-87a9-c151d255dfc7