Improve the error message when ACL is not attached

Check whether the ACL model is attached to a datasource and report a helpful message if it is not.

One of the reasons why the ACL model may not be attached is that the user did not call `loopback.autoAttach()`.

This is the original error message:

```
TypeError: Object function ModelConstructor(data, dataSource) {
  if (!(this instanceof ModelConstructor)) {
    return new ModelConstructor(data, dataSource);
  }
  if (ModelClass.settings.unresolved) {
    throw new Error('Model ' + ModelClass.modelName + ' is not defined.');
  }
  ModelBaseClass.apply(this, arguments);
  if (dataSource) {
    hiddenProperty(this, '__dataSource', dataSource);
  }
} has no method 'find'
```

/to: @raymondfeng or @ritch please review