Description of problem:

Due to various issues with systemd user instances, the admin may want to mask the user@.service unit to stop spawning systemd user instances.
When doing so, every time a user logs in, the following Error message is printed:

-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
Failed to start user service, ignoring: Unit user@<UID>.service is masked.
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

This is confusing to the admin and create false alarms.

This BZ is a request to backport Upstream's code, which basically hides the message when user@.service is masked:

-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
340 static void user_start_service(User *u) {
 :
352         r = manager_start_unit(u->manager, u->service, &error, &u->service_job);
353         if (r < 0)
354                 log_full_errno(sd_bus_error_has_name(&error, BUS_ERROR_UNIT_MASKED) ? LOG_DEBUG : LOG_WARNING, r,
355                                "Failed to start user service '%s', ignoring: %s", u->service, bus_error_message(&e    rror, r));
356 }
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------


Version-Release number of selected component (if applicable):

systemd-239


How reproducible:

Always


Steps to Reproduce:
1. Mask user@.service

# systemctl mask user@.service

2. Log in as a new user