extended error logging for the incomplete browserInit in case of children incomplete
Future improvements:
* added cleaning of callbacks that are known to be never be used later
* improved Future.require() debug name
Made Widget and Behaviour more robust when it comes to the quick-fast-switching between pages.
* Protected Behaviour::render() from two interfering calls for the same widget.
* Introduced rejection of widget's ready promise when incompletely initialized widget is going to "die".
* Refactored Widget::initBehaviour() to be pure "Futurized" and give correct feedback to it's caller in form of rejected promise when exception occurs. Also improved behaviour class check.
* Refactored widget ready promise to be Future.single(). This is more robust then new Future, because it can be accidentally completed in wrong place.
Improved debug info for incompleteBrowserInit warning a little
Merge fixes for incompleteBrowserInit