IndexedDB: Log exception messages

To detect regressions, log the |message| property of exceptions.

BUG=95723

Review URL: https://chromiumcodereview.appspot.com/19107003

git-svn-id: svn://svn.chromium.org/blink/trunk@154236 bbb929c8-8fbe-4397-9dbb-9b2b20218538