Deliver sendto() exceptions without disturbing control flow

This prevents the client from waking up every 200 ms while the network is down.
Addresses #243.

src/network/networktransport.h

@@ -102,6 +102,8 @@ namespace Network {
     unsigned int send_interval( void ) const { return sender.send_interval(); }
 
     const struct in_addr & get_remote_ip( void ) const { return connection.get_remote_ip(); }
+
+    const NetworkException *get_send_exception( void ) const { return connection.get_send_exception(); }
   };
 }