core/unit: fail earlier before spawning executor when we failed to realize cgroup

core/unit: fail earlier before spawning executor when we failed to realize cgroup

Before 23ac08115af83e3a0a937fa207fc52511aba2ffa, even if we failed to
create the cgroup for a unit, a cgroup runtime object for the cgroup is
created with the cgroup path. Hence, the creation of cgroup is failed,
execution of the unit will fail in posix_spawn_wrapper() and logged
something like the following:
```
systemd[1]: testservice.service: Failed to create cgroup /testslice.slice/testservice.service: Cannot allocate memory
systemd[1]: testservice.service: Failed to spawn executor: No such file or directory
systemd[1]: testservice.service: Failed to spawn 'start' task: No such file or directory
systemd[1]: testservice.service: Failed with result 'resources'.
systemd[1]: Failed to start testservice.service.
```

However, after the commit, when we failed to create the cgroup, a cgroup
runtime object is not created, hence NULL will be assigned to
ExecParameters.cgroup_path in unit_set_exec_params().
Hence, the unit process will be invoked in the init.scope.
```
systemd[1]: testservice.service: Failed to create cgroup /testslice.slice/testservice.service: Cannot allocate memory
systemd[1]: Starting testservice.service...
cat[1094]: 0::/init.scope
systemd[1]: testservice.service: Deactivated successfully.
systemd[1]: Finished testservice.service.
```
where the test service calls 'cat /proc/self/cgroup'.

To fix the issue, let's fail earlier when we failed to create cgroup.

Follow-up for 23ac08115af83e3a0a937fa207fc52511aba2ffa (v258).