Handle server rejection properly

[russelltg]

Repro:

cargo run -- udp://:1111 "srt://127.0.0.1:2222?passphrase=mypassasdaa&pbkeylen=16" &
cargo run -- "srt://:2222?passphrase=mypassasda&pbkeylen=16" -

gives

thread 'main' panicked at 'not yet implemented', /home/russell/srt-rs/srt-tokio/src/socket/listen.rs:37:29