Display friendly error message when spawning CGI process fails

```
If you try to run phpdesktop with PHP 5.6 on Windows XP you will get error 
message like this:

  Error 500: Internal Server Error
  Cannot spawn CGI process [C:\Documents and Settings\Czarek\Desktop\phpdesktop-chrome-31.7-php-5.6.1\www/index.php]: No such device or address
```

Original issue reported on code.google.com by `czarek.t...@gmail.com` on 7 Oct 2014 at 4:32
