alerting: fixes validation error when saving alerts in dash

If a panelId in the dashboard json is set to zero then the validation
silently fails. Instead of returning an error, it just ignores alerts and
saves the dashboard.

pkg/services/alerting/extractor.go

@@ -143,10 +143,15 @@ func (e *DashAlertExtractor) GetAlertFromPanels(jsonWithPanels *simplejson.Json)
 
 		// validate
 		_, err = NewRuleFromDBAlert(alert)
-		if err == nil && alert.ValidToSave() {
+		if err != nil {
+			return nil, err
+		}
+
+		if alert.ValidToSave() {
 			alerts = append(alerts, alert)
 		} else {
-			return nil, err
+			e.log.Debug("Invalid Alert Data. Dashboard, Org or Panel ID is not correct", "alertName", alert.Name, "panelId", alert.PanelId)
+			return nil, m.ErrDashboardContainsInvalidAlertData
 		}
 	}