Unify error messages

plainText.go

@@ -1,6 +1,6 @@
 package atom
 
-import "errors"
+import "fmt"
 
 type PlainText struct {
 	*CommonAttributes
@@ -15,15 +15,15 @@ func (p *PlainText) isText() bool { return true }
 // error.
 func (p *PlainText) Check() error {
 	if p.Type != "" && p.Type != "text" && p.Type != "html" {
-		return errors.New("type attribute of plain text must be text or html if not omitted")
+		return fmt.Errorf("type attribute of plain text %v must be text or html if not omitted", p)
 	}
 
 	if p.Type == "html" && !isCorrectlyEscaped(p.Text) {
-		return errors.New("text element of plain text not correctly escaped")
+		return fmt.Errorf("text element of plain text %v not correctly escaped", p)
 	}
 
 	if p.Text == "" {
-		return errors.New("text element of plain text empty")
+		return fmt.Errorf("text element of plain text %v empty", p)
 	}
 
 	return nil