unnest Lua exceptions when they get all the way back to main()

[Habbie]

Short description

Sometimes (like during startup), exceptions from C++ may bubble up via Lua all the way to main(). This PR improves error reporting for that situation.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master