accountcmd: return error instead of exit in loadBlsSecretKey (#444)

loadBlsSecretKey is called in `account generate-bls-proof` to support loading
BLS secret key from file. But this command also supports loading secret key from
keystore. In that case, no keyfile is given and this function should return
error so the command can handle the error and switch to load secret key from
keystore instead of exit.

cmd/ronin/accountcmd.go

@@ -516,7 +516,7 @@ func loadKeyManager(ctx *cli.Context) (*bls.KeyManager, []blsCommon.PublicKey, e
 
 func loadBlsSecretKey(ctx *cli.Context) (blsCommon.SecretKey, error) {
 	if ctx.Args().Len() != 1 {
-		utils.Fatalf("keyfile must be given as the only argument")
+		return nil, fmt.Errorf("keyfile must be given as the only argument")
 	}
 	keyfile := ctx.Args().First()
 	if len(keyfile) == 0 {