bark: Avoid panic on chain source edge case

Peterrequested to merge
bugfix/exit-tx-manager-crash into master
  • check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

This one failed the CI yesterday, I don't know why get_tx would return Ok(None) but it happened so I'd like to avoid a potential client crash by just returning an error instead.

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