xrootd4j: Update response codes in standalone server

Return kXR_NotFound where it makes sense.

Target: master, 1.2
Merge commit '4e23406cfa06beb9e3aa8dd95fa332e017b27402' into release/1.2

Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5816