Add onError event channel

[vitalets]

Currently adding error listener is possible only from original ws instance:

wsp.ws.addEventListener('error', listener);

There are two disadvantages:

1. Adding listener is only possible after connect (like in original WebSocket)
2. It is inconsistent with other event channels: onClose and onMessage

The solution is to add onError channel with following usage:

wsp.onError.addListener(e => console.error(e));