unit: garbage collect units with load error

Units that failed to load were never cleaned up. It was possible to
reach the 128K limit of units by attempting to load a bunch of nonsense.

Bug observed by Reartes Guillermo in
https://bugzilla.redhat.com/show_bug.cgi?id=680122

src/unit.c

@@ -858,6 +858,7 @@ fail:
         u->meta.load_state = UNIT_ERROR;
         u->meta.load_error = r;
         unit_add_to_dbus_queue(u);
+        unit_add_to_gc_queue(u);
 
         log_debug("Failed to load configuration for %s: %s", u->meta.id, strerror(-r));