Improve events (#454)

* Fix typo in docs

* Improve events
Expose `error` event
Add `once`
Allow method chaining for `on` and `once`

packages/rpc-provider/src/ws/index.ts

@@ -245,6 +245,7 @@ export default class WsProvider implements WSProviderInterface {
 
   private onSocketError = (error: Event): void => {
     l.debug(() => ['socket error', error]);
+    this.emit('error', error);
   }
 
   private onSocketMessage = (message: MessageEvent): void => {