Notify user about early errors via mw.notify

Errors which were early enough in the life cycle to completely
block the lightbox from displaying were so far invisible to the user.
We display them in a small popup using the core messaging functionality.

Change-Id: Id04fe5d3b54703e24cd216910b15cc8bae786434
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/414