title: How to access Rails's 'can't be blank' error message?
body: <p>What's the API to access Rails's default <code>can't be blank</code> error message when validation <code>:presence =&gt; true</code> fails?</p>
  
  <p>I have the model setup like this:</p>
  
  <pre><code>class TextDocument &lt; Document
    validate :content_not_blank   # 'content' is an attribute
  
    private
      def content_not_blank
        if content.blank? 
          errors.add(:content, ....?) # I want to access 'can't be blank'
        end
      end
  end
  </code></pre>