When a label fails to be generated, you'll get the event payload as below:
{
"event_type": "shipment.label.failed",
"resource_type": "shipment",
"resource_id": "ESTEST00000",
"label": {
"easyship_shipment_id": "ESTEST00000",
"platform_order_number": "#100001",
"status": "failed"
}
}