handle return value of 0 from recv in recv_loop
Issue raised in

https://github.com/aws/aws-nitro-enclaves-cli/pull/609

Replicating similar fix here.
Indicate count of bytes received and expected bytes in error string.

Documentation links:

https://docs.rs/nix/latest/nix/sys/socket/fn.recv.html
https://pubs.opengroup.org/onlinepubs/9699919799/functions/recv.html
https://man7.org/linux/man-pages/man2/recv.2.html