swap failure in the xmlrpc server.  To use it, the extra option
"extrainfo" needs to be set to true.  This will cause "value" to be a
structure (in the XML-RPC sence) with useful information instead of
just the exit value of the script.  The structure will contain at
least the following fields:
  cause: "temp", "user", etc
  cause_desc
  mesg: more specific error information
  exitval: script return value
  log: activity log
Note, that value may still be an integer in the case of some other
failure that is not swap related.

To support this, the "-X" option was added to swapexp and batchexp
which will output a RPC-XML method response to stdout when possible.