Suppress `ExternalLoginUserNotExist` error (#21504)

Fixes #21202
Closes #21276

An `ExternalLoginUser` is not mandatory if the current user account was
created with/by the external login source.

routers/web/auth/auth.go

@@ -6,6 +6,7 @@
 package auth
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"strings"
@@ -24,6 +25,7 @@ import (
 	"code.gitea.io/gitea/modules/session"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/modules/web/middleware"
 	"code.gitea.io/gitea/routers/utils"
@@ -619,9 +621,11 @@ func handleUserCreated(ctx *context.Context, u *user_model.User, gothUser *goth.
 	// update external user information
 	if gothUser != nil {
 		if err := externalaccount.UpdateExternalUser(u, *gothUser); err != nil {
+			if !errors.Is(err, util.ErrNotExist) {
 				log.Error("UpdateExternalUser failed: %v", err)
 			}
 		}
+	}
 
 	// Send confirmation email
 	if !u.IsActive && u.ID > 1 {

routers/web/auth/oauth.go

@@ -1068,8 +1068,10 @@ func handleOAuth2SignIn(ctx *context.Context, source *auth.Source, u *user_model
 
 		// update external user information
 		if err := externalaccount.UpdateExternalUser(u, gothUser); err != nil {
+			if !errors.Is(err, util.ErrNotExist) {
 				log.Error("UpdateExternalUser failed: %v", err)
 			}
+		}
 
 		if err := resetLocale(ctx, u); err != nil {
 			ctx.ServerError("resetLocale", err)