Cryptic "Bad Annotation" Error Message

A variable assignment with 2 `@extends` annotations gave this warning using NTI:

`WARNING - Bad type annotation. type annotation incompatible with other annotations. See https://github.com/google/closure-compiler/wiki/Bad-Type-Annotation for more information.
`
That could be a lot better (more helpful).

```js
/**
* @constructor
* @extends {Polymer.Element}
* @extends {ObservableModelsElement}
*/
const CardElement = observableModelsMixin(cardUtilsMixin(Polymer.GestureEventListeners(Polymer.Element)));
```