Make unregistered forward handlers fail silently (or with warning) but non-breaking

[bernd-wechner]

Currently if you declare a forwarder with forward.JavaScript the named handler must be registered on the client side with yl.registerForwardHandler. If it is not then when the DAL widget is clicked it will display Searching.... and never complete, but on the console will appear this error:

22:04:42.777 autocomplete_light.js:465 Uncaught TypeError: handler is not a function
    at Object.<anonymous> (autocomplete_light.js:465)
    at Function.each (jquery.js:381)
    at Object.yl.getForwards (autocomplete_light.js:438)
    at jQuery.fn.init.data (select2.js:60)
    at AjaxAdapter.query (select2.full.js:3620)
    at Select2.<anonymous> (select2.full.js:5669)
    at Select2.Observable.invoke (select2.full.js:655)
    at Select2.Observable.trigger (select2.full.js:645)
    at Select2.trigger (select2.full.js:5827)
    at Select2.open (select2.full.js:5851)
    at Select2.toggleDropdown (select2.full.js:5838)
    at DecoratedClass.<anonymous> (select2.full.js:5609)
    at DecoratedClass.Observable.invoke (select2.full.js:655)
    at DecoratedClass.Observable.trigger (select2.full.js:645)
    at HTMLSpanElement.<anonymous> (select2.full.js:1629)
    at HTMLSpanElement.dispatch (jquery.js:5429)
    at HTMLSpanElement.elemData.handle (jquery.js:5233)
(anonymous) @ autocomplete_light.js:465
each @ jquery.js:381
yl.getForwards @ autocomplete_light.js:438
data @ select2.js:60
AjaxAdapter.query @ select2.full.js:3620
(anonymous) @ select2.full.js:5669
Observable.invoke @ select2.full.js:655
Observable.trigger @ select2.full.js:645
Select2.trigger @ select2.full.js:5827
Select2.open @ select2.full.js:5851
Select2.toggleDropdown @ select2.full.js:5838
(anonymous) @ select2.full.js:5609
Observable.invoke @ select2.full.js:655
Observable.trigger @ select2.full.js:645
(anonymous) @ select2.full.js:1629
dispatch @ jquery.js:5429
elemData.handle @ jquery.js:5233

This should at worse issue a console warning and still function simply not calling (the unregistered) forward handler.

This makes for more robust application (the application I had in mind was to declare a handler by default on all my model forms, and only avail myself of it in JS on those I wanted to. But now I need the smarts both client and server side as to which fields will have a forwarder attached or not.