No error shown when config yaml does not exist for --use-tunnel-server

[pkbehera]

gnmic --config tunnel_server_config.yaml --use-tunnel-server subscribe -i 30s --path /debug/paramslog_level

Here tunnel_server_config.yaml does not exist in the current directory. gnmi starts and sit silent without showing any log or errors, making the user wonder: "is it even working?"

I built and used gnmic from v0.42.1, commit 6b35566