Improve error messages

Currently an invalid NetworkAssertion can be applied to a cluster and an error event is associated with the resource:

For example, leaving off the `path` in a secret volume projection Items:

```
Handler 'creation' failed with an exception. Will retry. Traceback (most recent call last): File "/home/brian/.cache/pypoetry/virtualenvs/netcheck-operator-A1Zto0ba-py3.11/lib/python3.11/site-packages/kopf/_core/actions/execution.py", line 276, in execute_handler_once result = await invoke_handler( ^^^^^^^^^^^^^^^^^^^^^ File "/home/brian/.cache/pypoetry/virtualenvs/netcheck-operator-A1Zto0ba-py3.11/lib/python3.11/site-packages/kopf/_core/actions/execution.py", line 371, in invoke_handl...lumes[1].secret.items[0].path: Required value, spec.template.spec.containers[0].volumeMounts[1].name: Not found: \"somecontext\"]","reason":"Invalid","details":{"name":"http-with-external-secret-data","group":"batch","kind":"Job","causes":[{"reason":"FieldValueRequired","message":"Required value","field":"spec.template.spec.volumes[1].secret.items[0].path"},{"reason":"FieldValueNotFound","message":"Not found: \"somecontext\"","field":"spec.template.spec.containers[0].volumeMounts[1].name"}]},"code":422}
```

Kopf also adds status information to the NetworkAssertion resource:

```
status:
kopf:
progress:
creation:
delayed: '2023-06-01T21:55:11.104245'
failure: false
message: >+
(422)

Reason: Unprocessable Entity

HTTP response headers: HTTPHeaderDict({'Audit-Id':
'd4766624-951e-4e92-bffe-79b12ec8e2ea', 'Cache-Control': 'no-cache,
private', 'Content-Type': 'application/json',
'X-Kubernetes-Pf-Flowschema-Uid':
'b7fe9208-0b6a-4b16-88fd-9f0110a73375',
'X-Kubernetes-Pf-Prioritylevel-Uid':
'2a031122-adc7-4705-ac5a-396a7b4d679a', 'Date': 'Thu, 01 Jun 2023
21:54:11 GMT', 'Content-Length': '667'})

HTTP response body:
{"kind":"Status","apiVersion":"v1","metadata":{},"status":"Failure","message":"Job.batch
\"http-with-external-secret-data\" is invalid:
[spec.template.spec.volumes[1].secret.items[0].path: Required value,
spec.template.spec.containers[0].volumeMounts[1].name: Not found:
\"somecontext\"]","reason":"Invalid","details":{"name":"http-with-external-secret-data","group":"batch","kind":"Job","causes":[{"reason":"FieldValueRequired","message":"Required
value","field":"spec.template.spec.volumes[1].secret.items[0].path"},{"reason":"FieldValueNotFound","message":"Not
found:
\"somecontext\"","field":"spec.template.spec.containers[0].volumeMounts[1].name"}]},"code":422}

purpose: create
retries: 2
started: '2023-06-01T21:53:10.897556'
success: false
```

The CRD could include better schema information to allow the validation to occur earlier.