Add compile-time tests for APT-based RequestFactory interface validator.
Move all validator message formatting to a single class.
Patch by: bobv
Review by: keertip,pquitslund, t.broyer
Suggested by: t.broyer

Review at http://gwt-code-reviews.appspot.com/1473801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10426 8db76d5a-ed1c-0410-87a9-c151d255dfc7