Allow retry on transient transfer failures

This patch fixes a minor regression introduced in 2.8 that prevents retry
on a transient error. The patch also removes retry on a permanent error.

Target: trunk
Request: 2.8
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6707/