processPipelineTo and error handling

["Schmitz, Jeffrey A" <Je...@boeing.com>]

Hello,
   I have a flowscript which calls processPipelineTo, and I can't figure out how to get back into the script if there is an exception in one of the components of the pipeline that has been called.  For some reason, any error handling in my sitemap sends its results back to the client (i.e. the browser) instead of back to the flowscript via  the specified parameter ( transResult in the below example)

                cocoon.processPipelineTo(importURL, null, transResult);

In the below handler, I was expecting the saxError.xml file contents to be sent back to my flowscript, but it ends up being sent to the browser instead, and my flowscript never continues on.  What am I missing here?

   <map:handle-errors>
      <map:select type="exception">
      <map:when test="sax-parser-error">
         <map:generate src="xml/saxError.xml" />
         <map:serialize type="xml" />
      </map:when>