@Override
  public byte[] getDeltaValue() {
    return delta;
  }
If an error occurred, return the Throwable, otherwise return null. If an error occurred, then
this event will be passed to the <code>onError</code> method of the CqListener instead of the
<code>onEvent</code> method.
geode-cq/src/main/java/org/apache/geode/cache/query/cq/internal/CqEventImpl.java