mod_report_forward: Fix traceback on error message passed to datetime function If archive:get() returns an error, e.g. because it can't find a message with the given ID, it may return e.g. nil, "item-not-found" or similar where the error condition ends up in reported_message_time, which gets passed to util.datetime, which thus ends up doing math to a string or error object.

<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>