Error messages in IE < 9

Refer to previous pull request: inossidabile/sprockets-preload/pull/4

It seems that I've mixed up two different errors in IE. The original error was an exception thrown by IE, because the item being deleted was not already in localStorage.

The second error is about handling custom events. I have removed the adjusted dispatching method for now and added a try-catch block to get rid of the error message in IE.
I haven't found any internal reference to the events "sprockets:progress" and "sprockets:loaded".

I'm fine with that if they are only an optional hook for displaying loading animations etc.