Panic in "If it does actually panic, then ping the authors of LiteBox"

Nobody has claimed this yet.

Trying to run `litebox_runner_linux_userland echo yay` on a stock desktop Ubuntu installation causes the following panic:

```
thread 'main' (41447) panicked at litebox_runner_linux_userland/src/lib.rs:118:48:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
```

Referring to this line:

https://github.com/microsoft/litebox/blob/86e26df9909b8adb7276f6bc039c5fa2da2ebba7/litebox_runner_linux_userland/src/lib.rs#L118

The function it's contained in has this comment:

https://github.com/microsoft/litebox/blob/86e26df9909b8adb7276f6bc039c5fa2da2ebba7/litebox_runner_linux_userland/src/lib.rs#L97-L99

I could not find any documentation on the expected environment setup, so adding that and/or a better error message would be very welcome.

Reproduce the panic with `litebox_runner_linux_userland echo yay` on a stock Ubuntu desktop, then inspect `litebox_runner_linux_userland/src/lib.rs` around lines 97-99 and 118. Determine the expected environment from the referenced function and improve the documentation and/or failure reporting so this invocation explains the missing requirement instead of panicking.

Written by the indexing model from the issue text.