pool: Fix error handling in staging code

A regression in 2.9. Causes pools to disable in case of staging
an existing file.

Target: trunk
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6990/
(cherry picked from commit c34fa7ef92e96d5131a7d338b28c8440e9340a87)
Merge pull request #553 from gbehrmann/fix/2.9/rb6990

Fix/2.9/rb6990