Test nw-err-xs0015-004

The pipeline

<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:ex="http://test" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
   <p:output port="result"/>
   <p:choose>
      <p:when test="true()">
      </p:when>
   </p:choose>
</p:declare-step>