Fixing the checksumPackage tests

These tests would raise a RuntimeBinderException - because of how ExposedObject works - rather than the specific exception. Let's do this in a cleaner way.

 - [x] internals available to test assembly
 - [x] specific exception can be handled by external caller
 - [x] added test for "file size different" path
 - [ ] `AwaitableAsyncSubject` should throw a custom exception when it doesn't have a result (so we know to read the inner exception)
 - [ ] update sample