From v0.20.0-rc3 (commit c6f458e478) onward
the ChainNotifier sub-server may still be initialising when clients attempt
to subscribe, currently resulting in a gRPC Unknown error with a plain-text
message. Change the notifier RPC endpoints to return codes.Unavailable instead
so clients can reliably interpret the condition as "retry later" and handle
the startup lag without unstable string matching.