notify: a failed Mattermost delivery's error names Mattermost (closes #227)

Mattermost is sent by the Slack sender, which wrapped every HTTP error
status in ErrSlackFailed, so a Mattermost endpoint answering 503 was
logged as "slack notification failed". The sender now takes the error
to wrap: the Slack endpoint passes ErrSlackFailed and the Mattermost
endpoint passes ErrMattermostFailed, which was defined but unused.

A new delivery test sets both endpoints to a stand-in server answering
503 and checks the error logged for each names its own endpoint.

Model: opus-5-5

internal/notify/export_test.go

@@ -85,10 +85,11 @@ func (svc *Service) SendNtfy(
 func (svc *Service) SendSlack(
 	ctx context.Context,
 	webhookURL *url.URL,
+	failed error,
 	title, message, priority string,
 ) error {
 	return svc.sendSlack(
-		ctx, webhookURL, title, message, priority,
+		ctx, webhookURL, failed, title, message, priority,
 	)
 }