Print loader exceptions in case AggregateCatalog would throw

Right now, if AggregateCatalog responsible for finding script packs throws, we would not show which assembly is missing. We can catch `ReflectionTypeLoadException` to remedy that - with this change, we get to display the failure DLL info correctly, for example:

![image](https://cloud.githubusercontent.com/assets/1710369/2561694/854b7d54-b824-11e3-92ac-e2762a24342d.png)