Don't WARN log UserNotExist errors on ExternalUserLogin failure (#16238)

Instead log these at debug - with warn logging for other errors.

Fix #16235

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/login_source.go

@@ -856,8 +856,12 @@ func UserSignIn(username, password string) (*User, error) {
 			return authUser, nil
 		}
 
+		if IsErrUserNotExist(err) {
+			log.Debug("Failed to login '%s' via '%s': %v", username, source.Name, err)
+		} else {
 			log.Warn("Failed to login '%s' via '%s': %v", username, source.Name, err)
 		}
+	}
 
 	return nil, ErrUserNotExist{user.ID, user.Name, 0}
 }