[FIXED JENKINS-13378] Remove XML string writer from IOException2 when XPath error occurs.

Remove XML string writer from IOException2 message when XPath error
occurs in /api/xml request.

The XML string writer has the potential to be very large (many
megabytes or even gigabytes) and these exceptions will be logged to
the default jenkins logfile by winstone. This has the potential to
quickly use all available disk space if, for example, a jenkins poller
(i.e. a chrome extension) makes frequent calls to the API that cause
errors.