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/