Test nw-err-xs0029-001

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:input port="source"/>
   <p:output port="result">
      <doc/>
   </p:output>
</p:declare-step>