Hhave swapexp/batchexp dump the error when the -w" option is
specified.  The error will look something like:

  ERROR:: <cause desc>

  <text of the error>

  Cause: <cause>
  Confidence: <confidence>

This will be the last thing printed.  The "::" is there to make
recognizing the error easy to scripts since they can just look for the
"ERROR::".