code: protected PersistException transformIntoPersistException(Throwable e) {
          if (e instanceof PersistException) {
              return (PersistException) e;
          }
          if (e instanceof InterruptedIOException) {
              return new PersistInterruptedException(e);
          }
          if (e instanceof FetchException) {
              return ((FetchException) e).toPersistException();
          }
          return null;
      }
docs: Override to support custom transformations, returning null if none is
  applicable. Be sure to call super first. If it returns non-null, return
  that result.
  
  @param e required exception to transform
  @return PersistException, or null if no applicable transform