pool: log with error if checksum scanner gets an IO error

Ticket: #8181
Acked-by: Karsten Schwank
Target: master, 2.9, 2.8, 2.7, 2.6
Require-book: no
Require-notes: no
(cherry picked from commit 5026cf9052b4170a54fc0c42389eba3ce40a52f3)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #661 from kofemann/fix-css-2.6

pool: log with error if checksum scanner gets an IO error