systemd-run: ensure error logs suggest to use '--user' when appropriate

Before:

$ systemd-run --service-type=notify --user false
Job for run-rc3fe52ee6ddd4a6eaaf1a20e0a949cdf.service failed because the control process exited with error code.
See "systemctl status run-rc3fe52ee6ddd4a6eaaf1a20e0a949cdf.service" and "journalctl -xeu run-rc3fe52ee6ddd4a6eaaf1a20e0a949cdf.service" for details.

After:

$ systemd-run --service-type=notify --user false
Job for run-r7791e380a7b6400ea01d6a0e5a458b23.service failed because the control process exited with error code.
See "systemctl --user status run-r7791e380a7b6400ea01d6a0e5a458b23.service" and "journalctl --user -xeu run-r7791e380a7b6400ea01d6a0e5a458b23.service" for details.

Fixes https://github.com/systemd/systemd/issues/21933

(cherry picked from commit 466f2351)