These APIs are primarily intended to let developers write custom 404 handlers. They can define an onBeforeHTTPError404() method on an Extension that gets added to Controller or RequestHandler.
The SS_HTTPResponse_Exception object has also been tidied up to override the status info of any SS_HTTPResponse object that might get passed. This is mainly to make it easier for callers (such as ContentController and ModelAsController) to use RequestHandler::httpError() more consistently.
This allows arguments to be passed along in an $allowed_actions deceleration of
the form 'action' => '->method' in the same way that arguments can be passed to
extension constructors when adding them using $extensions or
Object::add_extension.
I.e. 'action' => '->checkerMethod(false, 7, 2, "yesterday") would call the
checkerMethod method with the boolean false the numbers 7 and 2 and the string
"yesterday" as its arguments.
API CHANGE: Add DataList::newObject(), which creates a new object on that DataList.
API CHANGE: RequestHandler::handleRequest() now needs to handle a $model argument, if you override it.
