Extracts the error codes on a page and prints them so you can use them in your messages.properties

extract_error_codes.gsp

<g:each var="error" in="${myInstance.errors}">
  <g:each var="codes" in="${error?.fieldErrors?.codes}">
    <g:each var="code" in="${codes}">
       <div>${code}</div>
    </g:each>
  </g:each>
</g:each>