Exit after fatal dnsmasq errors

[DL6ER]

What does this implement/fix?

Fixes a bug introduced in #1881 where fatal dnsmasq config line errors can lead to FTL crashing after logging that there is a fatal event. The expected behavior is to cleanly return an error code instead of crashing which is restored by this PR.

This small fix is meant to go out as FTL v5.25.2

---

Related issue or feature (if applicable): Fixes #1942

Pull request in docs with documentation (if applicable): N/A

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pihole-ftl-suddenly-started-to-fail-on-docker/69933/9