code: @NotNull
      public Exceptional<T> or(@NotNull Supplier<Exceptional<T>> supplier) {
          if (throwable == null) return this;
  
          Objects.requireNonNull(supplier);
          return Objects.requireNonNull(supplier.get());
      }
docs: Returns current {@code Exceptional} if there were no exceptions, otherwise
  returns an {@code Exceptional} produced by supplier function.
  
  @param supplier  supplier function that produced an {@code Exceptional} to be returned
  @return this {@code Exceptional} if there were no exceptions, otherwise
  an {@code Exceptional} produced by supplier function
  @throws NullPointerException if {@code supplier} or its result is null