Fix issues with invalid Slack contact points (#41062)

* Add validation when creating/updating a contact point

* Change 201 status code for 200 (as it was before)

pkg/api/alerting.go

@@ -283,6 +283,10 @@ func CreateAlertNotification(c *models.ReqContext, cmd models.CreateAlertNotific
 		if errors.Is(err, models.ErrAlertNotificationWithSameNameExists) || errors.Is(err, models.ErrAlertNotificationWithSameUIDExists) {
 			return response.Error(409, "Failed to create alert notification", err)
 		}
+		var alertingErr alerting.ValidationError
+		if errors.As(err, &alertingErr) {
+			return response.Error(400, err.Error(), err)
+		}
 		return response.Error(500, "Failed to create alert notification", err)
 	}
 
@@ -301,6 +305,10 @@ func (hs *HTTPServer) UpdateAlertNotification(c *models.ReqContext, cmd models.U
 		if errors.Is(err, models.ErrAlertNotificationNotFound) {
 			return response.Error(404, err.Error(), err)
 		}
+		var alertingErr alerting.ValidationError
+		if errors.As(err, &alertingErr) {
+			return response.Error(400, err.Error(), err)
+		}
 		return response.Error(500, "Failed to update alert notification", err)
 	}