robustification around calling super.initialize() in annotators

add error messages to the standard base annotator classes that users extend, to cover the case where the user forgot to call super.initialize(). There are two fields that could be null if this super call isn't done. Instead of throwing NPE when they're referenced byt he base class impl, give an informative error message.

Fix the cases in our code where super.initialize() isn't called, including in our documentation.