Error Handling in Lens Library

[torbrenner]

In the PR #67 we had a short discussion about informing the end user about failed queries etc. As i noted there #67 (comment), i think the library part of lens should not directly send error messages to the user, as each application could have their own way of handling such events.
So i think lens should offer a way to send such messages to the end application e.g by emiting through the event api or alternatively by receiving a communication channel from the application.