10.50Status resource

A robust async API might return:

{
  "requestId": "REQ-123",
  "status": "ACCEPTED"
}

Then:

GET /requests/REQ-123

could return:

{
  "requestId": "REQ-123",
  "status": "COMPLETED"
}

or:

{
  "requestId": "REQ-123",
  "status": "FAILED",
  "errorCode": "CUSTOMER_INVALID"
}