Per the spec, it is required to serve light client bootstrap objects from finalized checkpoint block roots. Serving bootstrap objects for all other blocks is optional. Lighthouse only supports serving bootstrap objects for finalized checkpoint block roots. Currently, if a block root is used that isn't a finalized checkpoint block root the following error is returned

Sync committee branch for block root 0x7a734cd6e35b6c21f0cf55a8e3c36331a54f9774dface476be3b60d47e15f09a not found

We should instead return a better error message that informs the user that the provided block root is not a finalized checkpoint block root