How to raise an ActiveRecord::Rollback exception and return a value together?

I have a model that uses a acts_as_nested_set fork, and I've added a method to the model to save the model and move the node into the set in one transaction. This method calls a validation method to make sure the move is valid, which returns true or false. If the validation fails, I want my save method to raise ActiveRecord::Rollback to rollback the transaction, but also return false to the caller.

My model looks like this:

class Category < ActiveRecord::Base
  acts_as_nested_set :dependent => :destroy, :scope => :journal

  def save_with_place_in_set(parent_id)
    Category.transaction do
      return false if !save_without_place_in_set

      if !validate_move parent_id
        raise ActiveRecord::Rollback and return false
      else
        place_in_nested_set parent_id
        return true
      end
    end
  end

  alias_method_chain :save, :place_in_set

  def validate_move(parent_id)
    # return true or false if the move is valid
    # ...
  end

  def place_in_nested_set(parent_id)
    # place the node in the correct place in the set
    # ...
  end
end

However, when I call save in a situation that would fail, the transaction is rolled back but the function returns nil:

>> c = Category.new(:name => "test") 
=> #<Category id: nil, name: "test" parent_id: nil, lft: nil, rgt: nil>
>> c.save_with_place_in_set 47
=> nil
>> c.errors.full_messages
=> ["The specified parent is invalid"]