Previously when compiled without NDEBUG set it did, if a check
failed it called assert().  The tests checks for this using
ASSERT_DEATH.  However, ASSERT_DEATH leaks memory and this was
obscuring the valgrind output.  The changes here cause an exception
to be thrown (instead of a call to abort()) in the case of an error
and the unit tests now EXPECT_THROW instead of ASSERT_DEATH.