Fix Discord Webhook URL for invalid template (#44763)

This commit fixes an issue where an invalid template for Discord would change the Webhook URL to "" and cause "unsupported protocol scheme" errors.

pkg/services/ngalert/notifier/channels/discord.go

@@ -103,6 +103,7 @@ func (d DiscordNotifier) Notify(ctx context.Context, as ...*types.Alert) (bool,
 	u := tmpl(d.WebhookURL)
 	if tmplErr != nil {
 		d.log.Warn("failed to template Discord message", "err", tmplErr.Error())
+		return false, tmplErr
 	}
 
 	body, err := json.Marshal(bodyJSON)

pkg/services/ngalert/notifier/channels/discord_test.go

@@ -3,6 +3,7 @@ package channels
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"net/url"
 	"testing"
 
@@ -99,6 +100,14 @@ func TestDiscordNotifier(t *testing.T) {
 			settings:     `{}`,
 			expInitError: `failed to validate receiver "discord_testing" of type "discord": could not find webhook url property in settings`,
 		},
+		{
+			name: "Invalid template returns error",
+			settings: `{
+				"url": "http://localhost",
+				"message": "{{ template \"invalid.template\" }}"
+			}`,
+			expMsgError: errors.New("template: :1:12: executing \"\" at <{{template \"invalid.template\"}}>: template \"invalid.template\" not defined"),
+		},
 		{
 			name: "Default config with one alert, use default discord username",
 			settings: `{