code: @NotNull
      @SuppressWarnings("unchecked")
      public <E extends Throwable> Exceptional<T> ifExceptionIs(@NotNull Class<E> throwableClass, @NotNull Consumer<? super E> consumer) {
          if ( (throwable != null) &&
                  (throwableClass.isAssignableFrom(throwable.getClass())) ) {
              consumer.accept((E) throwable);
          }
          return this;
      }
docs: Invokes consumer function if exception class matches {@code throwableClass}.
  
  @param <E> the type of exception
  @param throwableClass  the class of an exception to be compared
  @param consumer  a consumer function
  @return an {@code Exceptional}