Use the interface name in generated errors

The codegen used to use the implementation names in the error messags.

  For example: "TypeError: Failed to execute 'atob' on 'DOMWindow' ...

Now we use the name as it is exposed to the DOM.

  For example: "TypeError: Failed to execute 'atob' on 'Window' ...

BUG=None

Review URL: https://codereview.chromium.org/25611006

git-svn-id: svn://svn.chromium.org/blink/trunk@158742 bbb929c8-8fbe-4397-9dbb-9b2b20218538