Improve error logging in toCeloFn (#2157)

This might help us debug
#2134 if it happens
again.

contracts/currency/currency.go

@@ -17,6 +17,7 @@
 package currency
 
 import (
+	"fmt"
 	"math/big"
 
 	"github.com/celo-org/celo-blockchain/common"
@@ -113,10 +114,10 @@ type ExchangeRate struct {
 // Requires numerator >=0 && denominator >= 0
 func NewExchangeRate(numerator *big.Int, denominator *big.Int) (*ExchangeRate, error) {
 	if numerator == nil || common.Big0.Cmp(numerator) >= 0 {
-		return nil, contracts.ErrExchangeRateZero
+		return nil, fmt.Errorf("numerator zero: %w", contracts.ErrExchangeRateZero)
 	}
 	if denominator == nil || common.Big0.Cmp(denominator) >= 0 {
-		return nil, contracts.ErrExchangeRateZero
+		return nil, fmt.Errorf("denominator zero: %w", contracts.ErrExchangeRateZero)
 	}
 	return &ExchangeRate{numerator, denominator}, nil
 }

miner/block.go

@@ -387,7 +387,10 @@ func createConversionFunctions(sysCtx *core.SysContractCallCtx, chain *core.Bloc
 		return sysCtx.GetGasPriceMinimum(feeCurrency)
 	}
 	toCeloFn := func(amount *big.Int, feeCurrency *common.Address) *big.Int {
-		curr, _ := currencyManager.GetCurrency(feeCurrency)
+		curr, err := currencyManager.GetCurrency(feeCurrency)
+		if err != nil {
+			log.Error("toCeloFn: could not get currency", "err", err)
+		}
 		return curr.ToCELO(amount)
 	}