fix(ngdoc/$moduleDocs): throw an error if a module is improperly tagged

Dgeni should throw a meaningful error if a module is improperly tagged as another entity (e.g. object, function, etc.).

At present, if a module has been improperly tagged as "object" or another entity, the following error is returned, but it does not provide any indication on how to fix the problem:

```
error:   TypeError: Cannot call method 'push' of undefined
    at C:\Data\test\grunt-udoc-sample\node_modules\dgeni-packages\ngdoc\processors\moduleDocs.js:52:31
    at Function.forEach (C:\Data\test\grunt-udoc-sample\node_modules\lodash\dist\lodash.js:3297:15)
    at Object.$process (C:\Data\test\grunt-udoc-sample\node_modules\dgeni-packages\ngdoc\processors\moduleDocs.js:40:9)
    at Q.then.catch.error.message (C:\Data\test\grunt-udoc-sample\node_modules\dgeni\lib\Dgeni.js:202:28)
    at _fulfilled (C:\Data\test\grunt-udoc-sample\node_modules\dgeni\node_modules\q\q.js:798:54)
    at self.promiseDispatch.done (C:\Data\test\grunt-udoc-sample\node_modules\dgeni\node_modules\q\q.js:827:30)
    at Promise.promise.promiseDispatch (C:\Data\test\grunt-udoc-sample\node_modules\dgeni\node_modules\q\q.js:760:13)
    at C:\Data\test\grunt-udoc-sample\node_modules\dgeni\node_modules\q\q.js:821:14
    at flush (C:\Data\test\grunt-udoc-sample\node_modules\dgeni\node_modules\q\q.js:108:17)
    at process._tickCallback (node.js:415:13)
error:   Error processing docs:
```