Add validator error handling to generate_vumark_instance

[adamtheturtle]

The try/except ValidatorError block was removed from generate_vumark_instance in both the requests mock server and Flask server because the branches were uncovered by tests.

Once tests exist that exercise invalid requests to POST /targets/{target_id}/instances, the error handling should be added back:

try:
    run_services_validators(
        request_headers=request.headers,
        request_body=_body_bytes(request=request),
        request_method=request.method or "",
        request_path=request.path_url,
        databases=self._target_manager.databases,
    )
except ValidatorError as exc:
    return exc.status_code, exc.headers, exc.response_text

[adamtheturtle]

The / block has been added back to in the requests mock server (via #2954). The Flask server handles this globally via . Closing as done.