Market Error

- add `ErrTooSmallOrder`

market.go

@@ -6,7 +6,8 @@ import (
 )
 
 var (
-	ErrMarketClosed = errors.New("market is closed")
+	ErrMarketClosed  = errors.New("market is closed")
+	ErrTooSmallOrder = errors.New("too small order size")
 )
 
 type Market interface {