Improve `gitlab-ctl replicate-geo-database` error messages

Users forgetting to run this command as root would not see any helpful warning messages. Now it will say:

* Stopping PostgreSQL and all GitLab services
warning: geo-postgresql: unable to open supervise/ok: access denied
warning: gitaly: unable to open supervise/ok: access denied
warning: gitlab-monitor: unable to open supervise/ok: access denied
warning: gitlab-workhorse: unable to open supervise/ok: access denied
warning: logrotate: unable to open supervise/ok: access denied
warning: nginx: unable to open supervise/ok: access denied
warning: node-exporter: unable to open supervise/ok: access denied
warning: postgres-exporter: unable to open supervise/ok: access denied
warning: postgresql: unable to open supervise/ok: access denied
warning: prometheus: unable to open supervise/ok: access denied
warning: redis: unable to open supervise/ok: access denied
warning: redis-exporter: unable to open supervise/ok: access denied
warning: sidekiq: unable to open supervise/ok: access denied
warning: unicorn: unable to open supervise/ok: access denied

[ERROR] Failed to execute: gitlab-ctl stop -- be sure to run as root

Closes gitlab-org/gitlab-ee#2176