units: do not fail if imds is not configured

units: do not fail if imds is not configured

In https://bugzilla.redhat.com/show_bug.cgi?id=2481466, it is reported
that systemd-imds-early-network.service fails on AWS EC2 instance with:

> No IMDS endpoint information provided or detected, cannot operate.

The service will be started on various systems where it doesn't have
enough configuration to run. The systemd-imds-generator only does some
superficial checks, so the tool should not mark the boot as failed
it turns out that IMDS is not supported.