Hmr dispose error message

You're not allowed to re-use a require after a hot update:

```js
var context = require.context(...);
var allTemplates = context.keys().map(context);
module.hot.accept(context.id, function() {
  // You are not allowed to reuse the context!
  allTemplates = context.keys().map(context);
});
```

When you hot-update twice, this will throw an error:

In the browser, on the second hot module replacement, I get an error:
```
    Uncaught TypeError: Cannot read property 'hot' of undefined
````

The error is thrown (in the browser) on this line:
```
   if(parent.hot._declinedDependencies[moduleId]) { 
````

So I've added an error to describe what happened. (A disposed parent was not re-installed after a hot code push. Hot update should re-install every diposed module!)