team: don't log failures to connect to teamd on error level

ensure_teamd_connection() is called from multiple spots. Sometimes we call opportunistically without having started teamd (e.g. when on update_connection() when generating a connection for teaming device that was created) and handle the failure to connect gracefully.

Let's not pollute the logs with things on ERROR level that are not actually serious. Demote the logging statement to DEBUG and log an ERROR only when we expect ensure_teamd_connection() to actually succeed.