private void checkForError() {
      if (callbackError != null) {
        throw new AssertionError("Exception occurred in callback", callbackError);
      }
    }
Returns wether or not one of this listener methods was invoked. Before returning, the
<code>invoked</code> flag is cleared.
geode-core/src/test/java/org/apache/geode/internal/DataSerializableJUnitTest.java