etcd: Don't log a Watch() error on context cancelation
During a normal shutdown we currently log an error after etcd Watch operations gets canceled:
Nov 06 16:25:49 lvs1013 liberica[55309]: time=2024-11-06T16:25:49.253Z level=ERROR msg="unable to watch" key=/conftool/v1/pools/eqiad/ncredir/nginx error="context canceled"