xrootd: Fix error message for read patch check

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7035/
(cherry picked from commit 1933780bcd0d1622497d246aec0d7aae2d21dcbd)
Merge pull request #615 from gbehrmann/fix/2.7/rb7035

Fix/2.7/rb7035