Revisit 503 errors for loading state

[bolasim]

Describe the bug
Loading states in a running truss server result in 503 error codes, which are problematic from a monitoring and alerting perspective (they are often bundled with 5xy codes which are used for alerting on-call).

We should find an appropriate error code (4xy level) here that works well for the liveness/readiness probes from a k8s side, but don't result in 5xy error codes.