NIFI-14975 fail Registry startup if nifi-registry-api handler is not available

[mosermw]

Summary

NIFI-14975

On startup, bad configuration or startup errors can cause NiFi Registry to successfully start and the web UI works, but Jetty can return 503 UNAVAILABLE errors on nifi-registry-api. One (of many) conditions this can happen is if the configuration is fine but an LdapUserGroupProvider or DatabaseUserGroupProvider cannot connect to external services.

This PR causes NiFi Registry to shutdown in these conditions. This aligns with how NiFi behaves in similar scenarios. It also works well in containerized environments, where an orchestrator can now notice Registry is not healthy and retry startup.

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  • JDK 21

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[exceptionfactory]

Thanks for this adjustment and aligning to the NiFi JettyServer approach @mosermw, looks good! +1 merging