install: never log from install functions

Instead, let the caller do that. Fix this by moving masked unit messages
into the caller, by returning a clear error code (ESHUTDOWN) by which
this may be detected.

src/core/dbus-manager.c

@@ -1651,6 +1651,8 @@ static int method_enable_unit_files_generic(
         scope = m->running_as == MANAGER_SYSTEM ? UNIT_FILE_SYSTEM : UNIT_FILE_USER;
 
         r = call(scope, runtime, NULL, l, force, &changes, &n_changes);
+        if (r == -ESHUTDOWN)
+                return sd_bus_error_setf(error, BUS_ERROR_UNIT_MASKED, "Unit file is masked");
         if (r < 0)
                 return r;
 
@@ -1885,6 +1887,8 @@ static int method_add_dependency_unit_files(sd_bus_message *message, void *userd
         scope = m->running_as == MANAGER_SYSTEM ? UNIT_FILE_SYSTEM : UNIT_FILE_USER;
 
         r = unit_file_add_dependency(scope, runtime, NULL, l, target, dep, force, &changes, &n_changes);
+        if (r == -ESHUTDOWN)
+                return sd_bus_error_setf(error, BUS_ERROR_UNIT_MASKED, "Unit file is masked");
         if (r < 0)
                 return r;