pool: Don't claim a restore was successfull when it failed

Addresses the issue that the pool logs "File successfully
restored from tape" no matter whether the restore was
successful or not.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5485/