title: RESTful service, how to respond if validation failed?
body: <p>I have service that takes some entity and needs to save/update this entity:</p>
  
  <pre><code>http://myhost.com/rest/entity
  </code></pre>
  
  <p>I use POST and submit JSON. Inside service it detects that entity passed is not good. Not valid, order passed in with customer that doesn not exist, etc.</p>
  
  <p>How should I reply? <code>HttpCode.NotFound</code>? Or others? How do you reply to such things?</p>