NullPointerException or ClassCastException if UPDATE or DELETE is performed on a diagnostic VTI

UPDATE and DELETE statements on read-only vtis will hit NullPointerException in insane mode, and ClassCastException in sane mode.

ij> delete from new org.apache.derby.diag.TransactionTable();
ERROR XJ001: Java exception: ': java.lang.NullPointerException'.

While these statements make no sense, a more graceful error should be returned, especially as in insane mode this causes the engine to shutdown. Code already exists to detect this situation but is not being called.

Found through adding negative tests for DERBY-571