chore: make test helper for log scraping more explicit during failure

[btlghrants]

Currently, when a pepr / a pepr-excellent-example fail in a particular way during testing, we get an abbreviated, rather-hard-to-understand / -diagnose error message out of the test runner.

    SyntaxError: Unexpected token '(', "(node:1) ["... is not valid JSON
        at JSON.parse (<anonymous>

The way we have to troubleshoot this is to pull the offending example down to execute locally (after injecting some add'l log dumps).

The ask here is: make the test helper that's throwing the opaque error message more explicit in the failure case. We want to:

• dump the whole error message / stack track, and
• dump out the logs that caused the error.

This would allow us to diagnose test issues without first having to pull it down & run it locally.