code: @Nullable
      public static <T> T log(Throwable cause) {
          logger.warn("Unexpected exception from a completion action:", cause);
          return null;
      }
docs: Logs the specified {@link Throwable}. For example:
  <pre>{@code
  CompletableFuture<?> f = ...;
  f.exceptionally(CompletionActions::log);
  }</pre>
  
  @return {@code null}