This diff introduces following changes to the Rust send_by_fcm_client function:

• As we don't use message_id anywhere at the upper level we should not return it;
• Properly handling errors by checking the results vector instead of the upper level only.

Related Linear task: ENG-1740

1. Successfully built using Docker.
2. Successfully call the function from C++:

#include "cxxbridge_code/src/lib.rs.h"
#include "rust/cxx.h
...
const std::string server_key = argv[1];
const std::string device_token = argv[2];
const std::string message_title = argv[3];
const std::string message_body = argv[4];
sendNotifToFCM(server_key, device_token, message_title, message_body);