Alerting: Fix so that sending an alert with the Alertmanager notifier doesn't fail when one of multiple configured URL's are down (#31079)

Fixes behaviour of Notify that returns error when one of the dispatch 
event return error, to maintain high availability, we should return error 
when all dispatched events return error instead.

Fixes #30509

pkg/services/alerting/notifiers/alertmanager.go

@@ -2,6 +2,7 @@ package notifiers
 
 import (
 	"context"
+	"fmt"
 	"regexp"
 	"strings"
 	"time"
@@ -170,6 +171,7 @@ func (am *AlertmanagerNotifier) Notify(evalContext *alerting.EvalContext) error
 
 	bodyJSON := simplejson.NewFromAny(alerts)
 	body, _ := bodyJSON.MarshalJSON()
+	errCnt := 0
 
 	for _, url := range am.URL {
 		cmd := &models.SendWebhookSync{
@@ -182,10 +184,15 @@ func (am *AlertmanagerNotifier) Notify(evalContext *alerting.EvalContext) error
 
 		if err := bus.DispatchCtx(evalContext.Ctx, cmd); err != nil {
 			am.log.Error("Failed to send alertmanager", "error", err, "alertmanager", am.Name, "url", url)
-			return err
+			errCnt++
 		}
 	}
 
+	// This happens when every dispatch return error
+	if errCnt == len(am.URL) {
+		return fmt.Errorf("failed to send alert to alertmanager")
+	}
+
 	return nil
 }