moved onError() method to IScreenReceiver from IPrimaryScreenReceiver.

also implemented onError in CClient which previously did not have
any way to handle display disconnection.

client/CServerProxy.cpp

@@ -221,6 +221,12 @@ CServerProxy::getOutputStream() const
 	return m_output;
 }
 
+void
+CServerProxy::onError()
+{
+	// ignore
+}
+
 void
 CServerProxy::onInfoChanged(const CClientInfo& info)
 {