mod_rest: Improve error handling during format mapping Prevents e.g. a nil, error return going directly into e.g. json encoding, resulting in "null" being returned. Further handling improvements down the line is needed.

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>