NN throws NPE if downgrade it during rolling upgrade from 3.x to 2.x

We can downgrade NN during roling upgrade (runned "hdfs dfsadmin -rollingUpgrade prepare) with HDFS-8432 involved. But with HDFS-14172 if the image has any unrecogized section, it will throw IOException at

private void loadInternal(......) {
    ......
    String n = s.getName();
    SectionName sectionName = SectionName.fromString(n);
    if (sectionName == null) {
      throw new IOException("Unrecognized section " + n);
    }
    ......
}

and throw NPE on Hadoop 2.x

private void loadInternal(......) {
    ......
    String n = s.getName();
    switch (sectionName)
    ......
}

When we downgrade NN from 3.x to 2.x, NN may load the image saved by 3.x NN. Then the lack of SectionName.ERASURE_CODING can break 2.x NN.
We should just skip the unrecogized section instead of throwing exception.