ENETDOWN is a legal error from TCP connect

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/tcp_connecter.cpp

@@ -296,7 +296,7 @@ zmq::fd_t zmq::tcp_connecter_t::connect ()
         errno = err;
         errno_assert (errno == ECONNREFUSED || errno == ECONNRESET ||
             errno == ETIMEDOUT || errno == EHOSTUNREACH ||
-            errno == ENETUNREACH);
+            errno == ENETUNREACH || errno == ENETDOWN);
 
         return retired_fd;
     }