RFR(L): 8218628: Add detailed message to NullPointerException describing what is null.

[Langer, Christoph]

> 122     private native void setDefaultMessage(String extendedMessage);
> Here, I agree with David, that you should do it in Java. Just use a shared
> secret, then you don't even need reflection...

Addition: you should not even need to use a shared secret as Throwable and NPE are in the same package. Just add some package private setter to Throwable's detailMessage.