Unify error messages

xhtmlDiv.go

@@ -2,7 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
+	"fmt"
 )
 
 type XHTMLDiv struct {
@@ -23,7 +23,7 @@ func NewXHTMLDiv(content string) *XHTMLDiv {
 // error.
 func (x *XHTMLDiv) Check() error {
 	if x.XMLNS != "http://www.w3.org/1999/xhtml" {
-		return errors.New("xmlns attribute of xhtml text must be http://www.w3.org/1999/xhtml")
+		return fmt.Errorf("xmlns attribute of xhtml text %v must be http://www.w3.org/1999/xhtml", x)
 	}
 
 	return nil