Suppress `ExternalLoginUserNotExist` error (#21504) (#21572)

Backport of #21504

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/auth/auth.go

@@ -613,9 +613,11 @@ 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 !user_model.IsErrExternalLoginUserNotExist(err) {
 				log.Error("UpdateExternalUser failed: %v", err)
 			}
 		}
+	}
 
 	// Send confirmation email
 	if !u.IsActive && u.ID > 1 {

routers/web/auth/oauth.go

@@ -1061,8 +1061,10 @@ func handleOAuth2SignIn(ctx *context.Context, source *auth.Source, u *user_model
 
 		// update external user information
 		if err := externalaccount.UpdateExternalUser(u, gothUser); err != nil {
+			if !user_model.IsErrExternalLoginUserNotExist(err) {
 				log.Error("UpdateExternalUser failed: %v", err)
 			}
+		}
 
 		if err := resetLocale(ctx, u); err != nil {
 			ctx.ServerError("resetLocale", err)