[foreign] RFR 8211277: Exception when dereferencing null pointer should be more specific

[Maurizio Cimadamore]

Hi,
this is a RFR for the changes discussed in [1] (again, thanks Jorn!). 
Some minor tweaks:

* consolidate the code paths for 'throwy' References - new 
Reference.OfGrumpy :-)

* tweaked test to check the exception cause - as otherwise the test 
passes even w/o the fix

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8211277/

Cheers
Maurizio

[1] - 
http://mail.openjdk.java.net/pipermail/panama-dev/2018-September/002796.html