title: REST: Which HTTP-status-codes?
body: <p>I have to handle some client requests to my server using rest.<br>
  The client makes a request sending an ID and I want to return suitable http-error-codes in case the ID</p>
  
  <ul>
  <li>is invalid</li>
  <li>doesn't exist</li>
  </ul>
  
  <p>But unfortunately I'm not sure about the right code.</p>