DERBY-6437 testLockingWithRollback failing on trunk with AssertionFailedError - isolation levels: 4 2

Added information about which exception caused the assertNull errors
in ConstraintCharacteristicsTest#executeThreads to help debugging.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1556780 13f79535-47bb-0310-9956-ffa450edef68