Added custom exception if a resource is not found and added name of requested resource to message.

It is a little bit hard to debug what is wrong if a resource is not found without knowing the name of the requested resource. I added a custom exception for that. Unfortunately, the property ResourceName of it gets cleared in XmlStackTraceException.GenerateStackedException but the information is in the exception message. Probably it might be better to remove ResourceNotFoundException.ResourceName.

Thanks a lot!