Unify error messages

xhtmlText.go

@@ -1,9 +1,6 @@
 package atom
 
-import (
-	"errors"
-	"fmt"
-)
+import "fmt"
 
 type XHTMLText struct {
 	*CommonAttributes
@@ -18,7 +15,7 @@ func (x *XHTMLText) isText() bool { return true }
 // error.
 func (x *XHTMLText) Check() error {
 	if x.Type != "xhtml" {
-		return errors.New("type attribute of xhtml text must be xhtml")
+		return fmt.Errorf("type attribute of xhtml text %v must be xhtml", x)
 	}
 
 	if err := x.XHTMLDiv.Check(); err != nil {