nspawn: consistenly fail if parsing the environment fails

nspawn: consistenly fail if parsing the environment fails

We would parse the environment twice (to re-apply settings after reading
config from disk), but we would not check the return code first time.
This means that for some settings we would ignore invalid values, while
for others, we'd fail at some point.

Let's just consistently fail. Those environment variables define important
aspects of behaviour, and it is better for the user if we ignore invalid
values. (Unknown settings are still ignored, so forward compatibility is
maintained.)