p:error outside p:try

Hello,

I have XProc with a bit of defensive coding like so:

<p:otherwise>
   <p:error code="ox:missing-source">
      <p:with-input>
         <message>Not finding expected source data at { $source-html-file } - try running GRAB-PLAYBOOK.xpl</message>
      </p:with-input>
   </p:error>
</p:otherwise>

This works in Morgana as I expect it to.

XML Calabash also throws the error, but like this:

Fatal ox:missing-source: [No explanatory message for ox:missing-source]
[with locator info]

What am I doing wrong or how do I get XML Calabash to report my message? (Maybe the element needs to be in a namespace?) Any hints or insights are welcome.

Thanks, Wendell