Prevent loop if there is an error in GetNotificationCount (#19799)

If the context is cancelled `.NotificationUnreadCount` in a template can
cause an infinite loop with `ctx.ServerError()` being called, which
creates a template that then calls `.NotificationUnreadCount` calling
`GetNotificationCount()` with the cancelled context resulting in an
error that calls `ctx.ServerError`... and so on...

This PR simply stops calling `ctx.ServerError` in the error handler code
for `.NotificationUnreadCount` as we have already started rendering and
so it is too late to call `ctx.ServerError`. Additionally we skip
logging the error if it's a context cancelled error.

Fix #19793

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

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

routers/web/user/notification.go

 package user
 
 import (
+	goctx "context"
 	"errors"
 	"fmt"
 	"net/http"
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/structs"
 )
 	c.Data["NotificationUnreadCount"] = func() int64 {
 		count, err := models.GetNotificationCount(c, c.Doer, models.NotificationStatusUnread)
 		if err != nil {
-			c.ServerError("GetNotificationCount", err)
+			if err != goctx.Canceled {
+				log.Error("Unable to GetNotificationCount for user:%-v: %v", c.Doer, err)
+			}
 			return -1
 		}