Make failed TUN socket operation non-fatal

It might be desirable to run the UE without superuser rights. More
concretely, it might run without NET_ADMIN. In this case, various
corresponding operations on the TUN socket fd might fail. In this
commit,

- improve error reports to make them more clear
- do not automatically exit(1), but return from the function to ensure
  continuity of the softmodem

openair1/SIMULATION/ETH_TRANSPORT/netlink_init.c

@@ -144,7 +144,7 @@ int netlink_init_tun(char *ifprefix, int num_if, int id) {//for UE, id = 1, 2, .
 
     if (nas_sock_fd[i] == -1) {
       LOG_E(PDCP, "TUN: Error opening socket %s (%d:%s)\n",ifname,errno, strerror(errno));
-      exit(1);
+      return 0;
     }
 
     LOG_I(PDCP, "TUN: Opened socket %s with fd nas_sock_fd[%d]=%d\n",