Add try/catch around cache checks

[TaterSalad]

Add try/catch blocks around cache access, so that if a cached element has been unloaded from the browser an uncaught exception won't be thrown. This typically happens when an IFRAME'd Element has been unloaded and a new Element or element with the same ID is requested.

See this post for details:
http://extjs.com/forum/showthread.php?t=70517

For now we have overridden Ext.get() static method and turned off garbage collection (since that method is in an anonymous / private function. We have implemented a copy of the function as an override. Kludgy.

Thanks.