Use 'ExceptionMessages::*' helper methods to generate CSSGroupingRule exceptions.

No real change, just cleaning up some code that landed before we settled
on the current approach.

Review URL: https://chromiumcodereview.appspot.com/23284002

git-svn-id: svn://svn.chromium.org/blink/trunk@156203 bbb929c8-8fbe-4397-9dbb-9b2b20218538