core/bpf-firewall: make failures in loading custom BPF program not critical

core/bpf-firewall: make failures in loading custom BPF program not critical

All other resource control features work as 'best-effort', and failures
in applying them are handled gracefully. However, unlike the other features,
we tested if the BPF programs can be loaded and refuse execution on failure.

Moreover, the previous behavior of testing loading BPF programs had
inconsistency: the test was silently skipped if the cgroup for the unit does
not exist yet, but tested when the cgroup already exists.

Let's not handle failures in loading custom BPF programs as critical, but
gracefully ignore them, like we do for the other resource control features.

Follow-up for fab347489fcfafbc8367c86afc637ce1b81ae59e.