manager: emit a message when we fail to create manager because /run is not set up

$ SYSTEMD_LOG_LEVEL=debug build/systemd --test --user
...
Failed to lookup RuntimeDirectory path: No such device or address   <---- this line is new
Failed to allocate manager object: No such device or address

We would fail and only say "Failed to allocate manager object: ENODEV" which is
not entirely self-explanatory. Let's add a better log message.