osdc/objecter: only log admin socket EEXIST for named clients

Commit 8f6e7a0cf14 ("osdc/objecter: Allow Objecter to store custom name
for admin socket") dropped the EEXIST arm of the registration check, so
that a failure to register objecter_requests would no longer be silently
swallowed.

That regressed callers which instantiate multiple librados clients in a
single process without setting a distinct name.  They now emit an error
on stderr, e.g. rbd device map prints

  -1 client.?.objecter error registering admin socket command: (17) File exists

on top of the real failure, which is misleading.  Such callers cannot
act on the message either: short lived CLI processes get no value from
registering a per-client admin socket command in the first place.

Restore the previous behaviour for unnamed clients, but keep it
discoverable by logging at debug level instead of silently ignoring it,
and continue to report a genuine error when two clients explicitly
register the same name via set_objecter_admin_socket_name().

Signed-off-by: Krunal Chheda <kchheda3@bloomberg.net>