[client]: simplify subscription API

[niklasad1]

The "current" subscription API for the client doesn't expose how a subscription was closed.

The subscription can return :
- None: The connection was closed
- None: The subscription could not keep up with server a.k.a slow-receiver problem.
- Err(e): Failed to parse the subscription item as T

Thus, we should modify the API to do the following:

• None: The connection was closed
• Err(SubscriptionError::Lagged) - The subscription lagged too far behind i.e, could not keep up with the server. The error itself indicates how many messages that were replace/missed
• Err(Subscription::Parse) - Failed to decode item as T

Some notes about SubscriptionError::Lagged

When this occurs it's up the user the decide whether that is okay to loose a few messages to continue, re-subscribe or drain/clear the buffer