Swallowing exceptions in client callbacks

[masojus]

When we call onSuccess()/onFailure on KeenCallback/KeenDetailedCallback we swallow any exceptions. It probably makes sense to not generate exceptions unless debug mode is turned on, if those exceptions were raised in the SDK, but if the exception was raised in client code, an argument could be made that clients would want to see that exception and fix it.

We should talk a bit more about this and decide what to do here.