Log AssertionErrors thrown by Generators to improve developer experience.
Fix typo in existing log message.
Patch by: bobv
Review by: scottb

Review at http://gwt-code-reviews.appspot.com/362801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7943 8db76d5a-ed1c-0410-87a9-c151d255dfc7