Change RemoteObject._describe method for Errors

Earlier this method returns class name for errors objects, now it returns className + ':' + error.message.

R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/401843002

git-svn-id: svn://svn.chromium.org/blink/trunk@178462 bbb929c8-8fbe-4397-9dbb-9b2b20218538