srm: Improve status messages of get requests

Addresses the issue that the description string of srmPrepareToGet
replies show the internal SRM scheduler state rather than the
description of the last request transition.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6148/