Fill in & warn against jQuery.Deferred.getStackHook

[mgol]

jQuery.Deferred.getStackHook is an old API with a new name at jQuery.Deferred.getErrorHook.

We should warn on getting/setting the old API & fill in the new one unless already set.

Ref jquery/jquery#5211
Ref jquery/api.jquery.com#1221

[mgol]

Oops; I forgot we already have #483 and I've even already submitted a PR for it. 😆