message: Better logging for CheckCoinSpend
  Also re-introduce the regtest specific conditional
diff: @@ -4684,10 +4684,10 @@ bool CWallet::CheckCoinSpend(libzerocoin::CoinSpend& spend, libzerocoin::Accumul
  {
  if (!spend.Verify(accumulator)) {
  receipt.SetStatus(_("The transaction did not verify"), ZPIV_BAD_SERIALIZATION);
  - return false;
  + return error("%s : The transaction did not verify", __func__);
  }
  - if (IsSerialKnown(spend.getCoinSerialNumber())) {
  + if (Params().NetworkID() != CBaseChainParams::REGTEST && IsSerialKnown(spend.getCoinSerialNumber())) {
  //Tried to spend an already spent zPIV
  receipt.SetStatus(_("The coin spend has been used"), ZPIV_SPENT_USED_ZPIV);
  uint256 hashSerial = GetSerialHash(spend.getCoinSerialNumber());
  