## Issue Addressed
Closes#3612
## Proposed Changes
- Iterates through BNs until it finds a non-optimistic head.
A slight change in error behavior:
- Previously: `spawn_contribution_tasks` did not return an error for a non-optimistic block head. It returned `Ok(())` logged a warning.
- Now: `spawn_contribution_tasks` returns an error if it cannot find a non-optimistic block head. The caller of `spawn_contribution_tasks` then logs the error as a critical error.
Co-authored-by: Michael Sproul <micsproul@gmail.com>