Should throw error on VertxGen declared inside VertxGen/DataObject

Currently, I've noticed a few generation errors regarding how classes and interfaces are annotated.

For example, if a `@VertxGen` interface has a `enum` declared inside, this will be valid java code and will compile and no warning is given by codegen. However, you will not be able to process such element.

```java
@VertxGen
interface A {
@VertxGen
enum E { }
}
```

In the example above `E` will never be processed and no warning or error is raised. I think we should raise an error in this situation.

The same applies for `@DataObject`. During the processing we should guarantee that this isn't allowed. This pattern is a problem to `es4x` but may not be for other generators, so we may agree in raising a warning instead.