Alerting: Return proper status code when trying to create alert notification channel with duplicate name or uid (#28043)

* Alerting: Return proper status code when trying to create an Alert Notification where the name or UID already exists.

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/api/alerting.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"errors"
 	"fmt"
 	"strconv"
 
@@ -273,6 +274,9 @@ func CreateAlertNotification(c *models.ReqContext, cmd models.CreateAlertNotific
 	cmd.OrgId = c.OrgId
 
 	if err := bus.Dispatch(&cmd); err != nil {
+		if errors.Is(err, models.ErrAlertNotificationWithSameNameExists) || errors.Is(err, models.ErrAlertNotificationWithSameUIDExists) {
+			return Error(409, "Failed to create alert notification", err)
+		}
 		return Error(500, "Failed to create alert notification", err)
 	}