pool: Improve xrootd error message on missing UUID

The existing error when the client doesn't present a valid UUID is misleading
as it usually states that the open request is not supported when it reallity
the request is not authorized due to the lack of the token.

This patch improves the error message and return code.

Target: trunk
Request: 2.10
Request: 2.11
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7515/
(cherry picked from commit 12d5aedef13860d3b875fcf135fa02ca2f4a59dc)
Merge pull request #1103 from gbehrmann/fix/2.10/rb7515

pool: Improve xrootd error message on missing UUID