Minor handling of wrong usage of Decorator

Little fix for http://bugzilla.qooxdoo.org/show_bug.cgi?id=7247 to show more verbose error message on wrong usage of Decorator.

```javascript
"table" : {
  decorator : [qx.ui.decoration.Single]
}
```

Originally this breaks the code with not very helpful error message. I have added this message instead while qx.debug is enabled:

*Invalid declaration of decorator table has been found. Only mixins can be enclosed in [] brackets. Found [Class qx.ui.decoration.Single].*