added - tx_graph::CalculateFeeError enum BREAKING CHANGES: changed - TxGraph::calculate_fee function to return Result<u64,CalculateFeeError> instead of Option<i64>
added - tx_graph::CalculateFeeError enum BREAKING CHANGES: changed - TxGraph::calculate_fee function to return Result<u64,CalculateFeeError> instead of Option<i64>