Ensure proper shutdown when crashing in a thread

[DL6ER]

What does this implement/fix?

Signal to the main process to shut down if a crash happens in a thread instead of trying to shut down cleanly from within this thread. Ensures that the non-success exit code is returned at the end, allowing systemd to try automatically restarting FTL.

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Related issue or feature (if applicable): Reported as part of #2307

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

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5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
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[yubiuser]

Build failed. And needs a rebase.

[DL6ER]

Not sure what happened here, probably too late yesterday... Rebased on latest development and force-pushed.