Add ErrAuthFailed

Convenience definition for backends.

Defined in https://www.rfc-editor.org/rfc/rfc4954#section-6

Author: emersion

README.md

@@ -107,7 +107,7 @@ type Session struct{}
 
 func (s *Session) AuthPlain(username, password string) error {
 	if username != "username" || password != "password" {
-		return errors.New("Invalid username or password")
+		return smtp.ErrAuthFailed
 	}
 	return nil
 }

backend.go

@@ -5,6 +5,11 @@ import (
 )
 
 var (
+	ErrAuthFailed = &SMTPError{
+		Code:         535,
+		EnhancedCode: EnhancedCode{5, 7, 8},
+		Message:      "Authentication failed",
+	}
 	ErrAuthRequired = &SMTPError{
 		Code:         502,
 		EnhancedCode: EnhancedCode{5, 7, 0},