Warn if a managed bean class is annotated as a session bean

It can happen that a managed bean class has a session bean annotation (e.g. @Stateless) when the class does not satisfy the requirements for the session bean class (see EJB 3.2 spec., 4.9.2 Session Bean Class).

For example, the following class is a managed bean as it is not public:

@Stateless
class Foo {}

However, a warning should be raised by the application server at the first place.