network: make GetHeaders best-effort

Be a nice neighbor, try to reply with whatever headers we have, don't fail
completely because of a single missing header.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/network/server.go

@@ -1120,7 +1120,7 @@ func (s *Server) handleGetHeadersCmd(p Peer, gh *payload.GetBlockByIndex) error
 		}
 		header, err := s.chain.GetHeader(hash)
 		if err != nil {
-			break
+			continue
 		}
 		resp.Hdrs = append(resp.Hdrs, header)
 	}