Don't create a wallet when wallet.dat already exists

cli.go

@@ -18,9 +18,14 @@ func (cli *CLI) createBlockchain(address string) {
 }
 
 func (cli *CLI) createWallet() {
-	wallet := NewWallet()
-	fmt.Printf("Your address: %s\n", wallet.GetAddress())
-	wallet.SaveToFile()
+	wallet, err := NewWallet()
+	if err == nil {
+		fmt.Printf("Your address: %s\n", wallet.GetAddress())
+		wallet.SaveToFile()
+	} else {
+		fmt.Println(err.Error())
+		os.Exit(1)
+	}
 }
 
 func (cli *CLI) getBalance(address string) {

wallet.go

@@ -6,8 +6,10 @@ import (
 	"crypto/rand"
 	"crypto/sha256"
 	"encoding/gob"
+	"errors"
 	"io/ioutil"
 	"log"
+	"os"
 
 	"golang.org/x/crypto/ripemd160"
 )
@@ -58,11 +60,14 @@ func (w Wallet) SaveToFile() {
 }
 
 // NewWallet creates and returns a Wallet
-func NewWallet() *Wallet {
+func NewWallet() (*Wallet, error) {
+	if _, err := os.Stat(walletFile); !os.IsNotExist(err) {
+		return nil, errors.New("Wallet already exists")
+	}
 	private, public := newKeyPair()
 	wallet := Wallet{private, public}
 
-	return &wallet
+	return &wallet, nil
 }
 
 func newKeyPair() ([]byte, []byte) {