nfs: propagate nfs exception on readdir

do not convert any exception to IO error. Current
behavior reported access deny as io error.

Acked-by: Paul Millar
Target: master, 0.7, 0.6
(cherry picked from commit 7ba3337e2716a6d181872243ad80e9b1d44b99c5)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>