proposal: add new `err` parameter to `connectHandler`

[acouvreur]

The current connectHandler has the following signature:

func(device Device, connected bool)

In case of connection failure or unexpected disconnection, we only have the connected boolean value. But more information can be exposed through an error object.

After a quick experimentation I was able to retrieve the following disconnection error message: The connection has timed out unexpectedly. from the underlying stack, and I think this is really worth the change.

My proposal, is to update the connect handler with an error parameter:

func(device Device, connected bool, err error)

[HattoriHanzo031]

I didn't yet go into consideration wether breaking change is justified here, but regardless, if it is decided to make the change, maybe consider making it a bit more future proof by using a struct that can have both connected status and error value instead of separate arguments.