Add user-overrideable GtExceptionHandler class

Also: the code to call the class if an exception reaches the top of
the call stack without being handled.
Remove redundant try/catch from DomEl.
Add length property to EmptyObject.
Change HttpError data check to typesafe
Merge branch '8w-improved-exception-handling' of github.com:8w/PHP.Gt into PR-8w-improved-exception-handling

Conflicts:
	Framework/EmptyObject.php
Merge branch 'PR-8w-improved-exception-handling'