Be more informative if loopback setup fails

We're seeing loopback setup fail in Fedora infrastructure for rpm-ostree
composes: https://pagure.io/releng/issue/6602

This should help debug.  Tested with strace fault injection:

```
$ strace -o strace.log -f -e fault=bind:error=EPERM bwrap --unshare-net --bind / / true
loopback: Failed to bind NETLINK_ROUTE socket: Operation not permitted
$
```

Closes: #166
Approved by: alexlarsson

bubblewrap.c

@@ -1911,8 +1911,8 @@ main (int    argc,
    */
   switch_to_user_with_privs ();
 
-  if (opt_unshare_net && loopback_setup () != 0)
-    die ("Can't create loopback device");
+  if (opt_unshare_net)
+    loopback_setup (); /* Will exit if unsuccessful */
 
   ns_uid = opt_sandbox_uid;
   ns_gid = opt_sandbox_gid;

network.c

@@ -132,7 +132,7 @@ rtnl_setup_request (char  *buffer,
   return (struct nlmsghdr *) header;
 }
 
-int
+void
 loopback_setup (void)
 {
   int r, if_loopback;
@@ -148,15 +148,15 @@ loopback_setup (void)
 
   if_loopback = (int) if_nametoindex ("lo");
   if (if_loopback <= 0)
-    return -1;
+    die_with_error ("loopback: Failed to look up lo");
 
   rtnl_fd = socket (PF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_ROUTE);
   if (rtnl_fd < 0)
-    return -1;
+    die_with_error ("loopback: Failed to create NETLINK_ROUTE socket");
 
   r = bind (rtnl_fd, (struct sockaddr *) &src_addr, sizeof (src_addr));
   if (r < 0)
-    return -1;
+    die_with_error ("loopback: Failed to bind NETLINK_ROUTE socket");
 
   header = rtnl_setup_request (buffer, RTM_NEWADDR,
                                NLM_F_CREATE | NLM_F_EXCL | NLM_F_ACK,
@@ -178,7 +178,7 @@ loopback_setup (void)
   assert (header->nlmsg_len < sizeof (buffer));
 
   if (rtnl_do_request (rtnl_fd, header) != 0)
-    return -1;
+    die_with_error ("loopback: Failed RTM_NEWADDR");
 
   header = rtnl_setup_request (buffer, RTM_NEWLINK,
                                NLM_F_ACK,
@@ -194,7 +194,5 @@ loopback_setup (void)
   assert (header->nlmsg_len < sizeof (buffer));
 
   if (rtnl_do_request (rtnl_fd, header) != 0)
-    return -1;
-
-  return 0;
+    die_with_error ("loopback: Failed RTM_NEWLINK");
 }

network.h

@@ -18,4 +18,4 @@
 
 #pragma once
 
-int loopback_setup (void);
+void loopback_setup (void);