Handling of subscription failures

[bmancini55]

This issue is to put together a strategy for handling subscription failures.

Right now, when there is a bad subscription, CCXWS does not uniformly report that the subscription has failed. This issue will address this gap and make changes to accurately report to the client that a subscription could not be completed.

Catalog by exchange how subscription errors are transmitted to the client. Some clients will send error messages back to the client.

Determine the proper course of action to take when a subscription has failed. This is likely going to be internally cleaning up the subscription map and emitting an event. Note that in PR #101, there is a discussion about REST failures occurring during subscriptL2Update calls. With the Binance client, if unsubscribe is called, it will trigger a reconnection event of the socket. Solutions should be mindful of the global picture.

Determine a testing strategy for testing invalid symbols or other subscription failures.