Error logging on payload build failure and graceful cancellation token handling

[avalonche]

If the payload building fails the flashblock_cancel_token_tx gets dropped which causes a send error when trying to send a child cancellation token in flashblocks. There is an error log indicating the channel has closed which is not helpful is diagnosing payload build failures.

if let Some(Err(err)) = cancelled {
    error!(target: "payload_builder", "Timer task encountered error: {err}");
}