Clean up ContainerNode exception messages.

The externally visible strings should read 'Node'; 'ContainerNode' is an impl. detail.

BUG=270033

Review URL: https://chromiumcodereview.appspot.com/24982002

git-svn-id: svn://svn.chromium.org/blink/trunk@158430 bbb929c8-8fbe-4397-9dbb-9b2b20218538