This test sinks its entire content, and thus the input which would generate the expected error need not be evaluated.

   <p:declare-step version="1.0" xmlns:p="http://www.w3.org/ns/xproc">
     <p:sink>
       <p:input port="source">
         <p:document href="unsupported://foo/bar.xml" />
       </p:input>
     </p:sink>
   </p:declare-step>