#428 refactor: grab stderr for Linux

This didn't matter on Windows since over there all errors seem to go through `stdout` :unamused: but on Linux things behave as you might expect with errors going to `stderr`.

(I've now realised that this is what threw me when I was initially trying to get things working and I thought that `stdout` capture with `System.Diagnostics.Process` was broken on Linux!)

#428