minor: added ErrNilQuery and ErrNilRow

Author: ralvarezdev

sql/errors.go

@@ -0,0 +1,10 @@
+package sql
+
+import (
+	"errors"
+)
+
+var (
+	ErrNilQuery = errors.New("sql query cannot be nil")
+	ErrNilRow   = errors.New("sql row cannot be nil")
+)