Normally $exceptionHandler doesn't throw an exception. It is normally
used just for logging and so on. But if an application developer
implemented a version that did throw an exception then $q would never
have called reject() when converting an exception thrown inside a `then`
handler into a rejected promise.
It is now possible to notify a promise through deferred.notify() method.
Notifications are useful to provide a way to send progress information
to promise holders.
When waiting for several promises at once, it is often desirable to
have them by name, not just by index in array.
Example of this kind of interface already implemented would be a
$routeProvider.when(url, {resolve: <hash of promises>}), where
resources/promises are given by names, and then results accessed
by names in controller.
