event: resubscribe with error handler

[lukasz-zimnoch]

Added a possibility to pass a custom callback function which will be invoked on subscription error.

This change will allow clients to receive the original subscription error and perform some additional actions like logging to make debugging easier.

[fjl]

Thanks for this change. It looks very useful, but I think we can do it with just one callback. The ResubscribeFunc is called every time the subscription needs to be created, so you can already hook into it and log. We cannot change the signature of ResubscribeFunc, but could make a new type like

// A ResubscribeErrFunc attempts to establish a subscription.
// For every call but the first, the second argument to this function is
// the error that occurred with the previous subscription.
type ResubscribeErrFunc func(context.Context, error) (Subscription, error)

I'm mostly suggesting this because I want to avoid having two callback parameters
when one is enough.

[lukasz-zimnoch]

Thanks for the review @fjl. I changed the code according to your suggestion.