Make mavsdk_server API consistent

[julianoes]

The mavsdk_server API mixes bool and int. I think we can improve it.

MAVSDK/src/mavsdk_server/src/mavsdk_server_api.cpp

Lines 10 to 25 in fa2c26f

	int mavsdk_server_run(
	MavsdkServer* mavsdk_server, const char* system_address, const int mavsdk_server_port)
	{
	if (!mavsdk_server->connect(std::string(system_address))) {
	// Connection was cancelled
	return false;
	}
	auto grpc_port = mavsdk_server->startGrpcServer(mavsdk_server_port);
	if (grpc_port == 0) {
	// Server failed to start
	return false;
	}
	return true;
	}

And we should also docstring document it while at it.