Handle error in round robin.

Fixes #151
Currently we cannot deliver same message to another peer cause it may be
already partially sent down the wire. So we just disconnect this peer
from the list and return an Err.

src/backend.rs

@@ -69,8 +69,9 @@ impl GenericSocketBackend {
                             self.round_robin.push(next_peer_id.clone());
                             return Ok(next_peer_id);
                         }
-                        Err(_) => {
-                            panic!("Don't know how to handle this now");
+                        Err(e) => {
+                            self.peer_disconnected(&next_peer_id);
+                            return Err(e.into());
                         }
                     };
                 }