Don't silenty exit on failure to open /proc/net/{ip,ip6}_tables_names

Victor Stinner <victor.stinner@inl.fr>
diff --git a/iptables-save.c b/iptables-save.c
index a8a17aa..f020113 100644
--- a/iptables-save.c
+++ b/iptables-save.c
@@ -242,7 +242,9 @@
 
 	procfile = fopen("/proc/net/ip_tables_names", "r");
 	if (!procfile)
-		return 0;
+		exit_error(OTHER_PROBLEM,
+			   "Unable to open /proc/net/ip_tables_names: %s\n",
+			   strerror(errno));
 
 	while (fgets(tablename, sizeof(tablename), procfile)) {
 		if (tablename[strlen(tablename) - 1] != '\n')