Confusing CI message when support for older pack format is broken

[peterwaller-arm]

If you break elfshaker such that it can't read a previous version of the pack, it fails with no error messages, only saying 'Script exited with error, tempfiles preserved in /dev/shm/test-T20211114155347'.

See:

https://github.com/elfshaker/elfshaker/runs/4204284944?check_suite_focus=true

I think we should improve this somehow.