Add PoolL2Tx.Info about the status of the tx

PoolL2Tx.Info contains information about the status & State of the
transaction. As for example, if the Tx has not been selected in the last
batch due not enough Balance at the Sender account, this reason would
appear at this parameter.
This will help the client (wallet, batchexplorer, etc) to reason why a
L2Tx is not selected in the forged batches.

txprocessor/txprocessor.go

@@ -1300,16 +1300,19 @@ func (tp *TxProcessor) computeEffectiveAmounts(tx *common.L1Tx) {
 }
 
 // CheckEnoughBalance returns true if the sender of the transaction has enough
-// balance in the account to send the Amount+Fee
-func (tp *TxProcessor) CheckEnoughBalance(tx common.PoolL2Tx) bool {
+// balance in the account to send the Amount+Fee, and also returns the account
+// Balance and the Fee+Amount (which is used to give information about why the
+// transaction is not selected in case that this method returns false.
+func (tp *TxProcessor) CheckEnoughBalance(tx common.PoolL2Tx) (bool, *big.Int, *big.Int) {
 	acc, err := tp.s.GetAccount(tx.FromIdx)
 	if err != nil {
-		return false
+		return false, nil, nil
 	}
 	fee, err := common.CalcFeeAmount(tx.Amount, tx.Fee)
 	if err != nil {
-		return false
+		return false, nil, nil
 	}
 	feeAndAmount := new(big.Int).Add(tx.Amount, fee)
-	return acc.Balance.Cmp(feeAndAmount) != -1 // !=-1 balance<amount
+	return acc.Balance.Cmp(feeAndAmount) != -1, // !=-1 balance<amount
+		acc.Balance, feeAndAmount
 }