DOMException should be subclass of Error

Per http://heycam.github.io/webidl/#es-exception-objects and http://heycam.github.io/webidl/#es-DOMException-prototype-object DOMException should be a subclass of Error and therefore have `name` and `message` properties.

Sample:

``` js
/** @param {DOMException} error */
function showError(error) {
console.log(error.name);
console.log(error.message);
}
```

Closure Compiler service reports:

```
JSC_INEXISTENT_PROPERTY: Property name never defined on DOMException at line 3 character 20
console.log(error.name);
^
JSC_INEXISTENT_PROPERTY: Property message never defined on DOMException at line 4 character 20
console.log(error.message);
^
```

Also note that `code` is intended to be an own-property not necessarily a getter on the prototype, but this is not clearly specified at the moment, and browsers (e.g. Chrome) may implement it as an inherited getter.