[#364] raise error when previous hash is None + fix indentation
[#364] raise error when previous hash is None + fix indentation

CounterpartyXCP/counterparty-lib,monaparty/counterparty-lib,Ziftr/counterpartyd,monaparty/counterparty-lib,ClearingHouse/clearinghoused,coinwarp/dogeparty-lib,ClearingHouse/clearinghoused,Ziftr/counterpartyd,tokenly/counterparty-lib,F483/counterparty-lib,jarymoth/counterpartyd,CounterpartyXCP/counterparty-lib,stefcrypto/counterparty-lib,F483/counterparty-lib,gotcha/testcounter