Fixed problem when trying to load already loaded asset

Hi!

This patch fixes problem with refactored loader function that occurs when trying to (re)download already loaded assets.

Couple months ago I sent a PR with a fix for this, which made sure the file wouldn't be redownloaded, and still would call the onComplete function - but it wouldn't call the onError function in this specific situation. 

This patch now does all that: avoids redownloading the given asset, calls the onError function, and properly calls onComplete when finished. 

I messed that first PR when trying to push (squashing) this updated version, so please forgive me for sending it again.
@kevinsimper Do you still think that's not right? 
The fact that you get the error messages in the fiddle with the fix means ```err``` is being called in line 337.