systemctl: print better message if default target is masked

If the default target is masked, `systemctl get-default` prints

  Failed to get default target: Operation not possible due to RF-kill

That's a bit too cryptic, so let's make it clear what's actually
happening.

Fixes #26589.

src/core/dbus-manager.c

@@ -2200,6 +2200,8 @@ static int method_get_default_target(sd_bus_message *message, void *userdata, sd
                 return r;
 
         r = unit_file_get_default(m->unit_file_scope, NULL, &default_target);
+        if (r == -ERFKILL)
+                sd_bus_error_setf(error, BUS_ERROR_UNIT_MASKED, "Unit file is masked.");
         if (r < 0)
                 return r;
 

src/systemctl/systemctl-set-default.c

@@ -50,6 +50,8 @@ static int determine_default(char **ret_name) {
 
         if (install_client_side()) {
                 r = unit_file_get_default(arg_scope, arg_root, ret_name);
+                if (r == -ERFKILL)
+                        return log_error_errno(r, "Failed to get default target: Unit file is masked.");
                 if (r < 0)
                         return log_error_errno(r, "Failed to get default target: %m");
                 return 0;