CreateProcess wrapper's error message is bad

In stout/os/windows/shell.hpp: Try<ProcessData> create_process(...) wrapper:

We have an `arg_string` and an `arg_buffer` because of oddities of `CreateProcessW`. But when composing the error message we need to use the string, not the buffer. Otherwise stringify kindly outputs a massive array of bytes. Currently by mistake we use the buffer, probably due to a renaming refactor I did.