Update nob_file_exists to distinguish between "does not exists" and other windows errors (todo).

Couldn't get nob to compile and thought #if _WIN32 was the cause, forked already, and then noticed that it doesn't matter, it was something on my end.
So I decided to just go ahead and fix something, and #ifdef _WIN32 is now just a consistancy/cleanliness bonus.
Had some trouble with FormatMessage, so helped myself to the answer from stackoverflow.com/questions/455434

Just made a little function and started blasting.

Upgrade windows error handling

That wasn't supposed to be in there...

When implementing previous changes to windows error reporting, exit status of user ran commands would get ran through nob_get_last_windows_error_name and return erroneous windows error messages.