title: Adding data alongside model.errors.add
body: <p>I'm looking to send along some data alongside errors.  I am guessing it's somewhere in the responders, though not a clue where to start.  In essence, I want to do</p>
  
  <pre><code>model.errors.add :some_attr, "error message"
  model.also_send_attrs = { prop: "some data also sent" }
  </code></pre>
  
  <p>and have the structure be the usual (with some appends)</p>
  
  <pre><code>{ 
    errors: { some_attr: ['error message'] },
    also_send_data: { prop: "some data also sent" }
  }
  </code></pre>
  
  <p>so is it possible to send data alongside an error, if one occurs?</p>