Instance deletion produces incorrect status codes

[teisenbe]

Instance deletion returns 404 if the target instance exists and is neither stopped nor failed. It should probably return either 400 or one of the precondition failure status codes

To reproduce, run this from the root of the omicron repo:

tools/oxapi_demo project_create_demo p1
tools/oxapi_demo instance_create_demo p1 i1
tools/oxapi_demo instance_reboot p1 i1
tools/oxapi_demo instance_delete p1 i1

[davepacheco]

@smklein can you take a look at this? I think this regressed with #192. We used to handle this case explicitly and it did produce the message:

$ ./tools/oxapi_demo instance_delete p1 i1
++ curl -sSi http://127.0.0.1:12220/projects/p1/instances/i1 -X DELETE
++ json -ga
HTTP/1.1 400 Bad Request
content-type: application/json
x-request-id: f711a757-9ca7-43cd-905b-5dfaef8492b1
content-length: 156
date: Wed, 20 Oct 2021 22:22:21 GMT

{
  "request_id": "f711a757-9ca7-43cd-905b-5dfaef8492b1",
  "error_code": "InvalidRequest",
  "message": "instance cannot be deleted in state \"running\""
}

We should also add a test to test_instances for this.

[smklein]

Fix is up in #329