Get the location of the electron logfiles from the logger itself

[davemfish]

Instead of assuming we know where the logfiles are, ask the logger.
Fixes #1452

There's also some additional logging here on the exit of an invest subprocess.

Checklist

• Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the Workbench UI (if relevant)

[emlys]

Thanks, @davemfish! I tested this on my mac and it works for me.