Test that exceptions are not hidden by aggregation
Try harder to diag() errors from 'do $file'
Check if $@ is a ref before comparing it to our ref

This avoids a warning in the tests:
Argument ... isn't numeric in numeric eq (==) at lib/Test/Aggregate.pm line 364.
Clean up some cruft from the new test script

* Remove debug noise
* fix skip number in new test
* fix a comment