lib/deploy: Ignore errors from FITHAW

lib/deploy: Ignore errors from FITHAW

In the production case since we used `daemon()` our stderr is `/dev/null`¹
there's not much use in logging errors from `FITHAW` or `exit(1)`, and doing so
breaks the test suite which checks the return from `waitpid()`. There's nothing
we can really do if `FITHAW` fails, and in most of those cases `EINVAL`,
`EOPNOTSUPP`, we *shouldn't* do anything anyways.

¹ Though perhaps we should set up the systemd journal, but let's not
  go there right now.

Closes: #1084
Approved by: jlebon