Handle potential serialization errors in FinalizedEpoch::insert_signature

[coderabbitai]

The method FinalizedEpoch::insert_signature uses unwrap() on the result of bincode::serialize, which can cause a panic if serialization fails. It's better to handle this error to prevent unexpected crashes and improve the robustness of the code.

This issue was identified during discussion in PR #123 by @coderabbitai[bot]. Please refer to the comment here for more details.

Requested by @distractedm1nd.