Display a friendly message when the path to php-cgi or the www directory is invalid

```
When the path to php-cgi.exe is invalid you will see in browser such a message:

  Error 500: Internal Server Error
  Cannot spawn CGI process [D:\git_test\phpdesktop-chrome\www/index.php]: No such device or address
```

Original issue reported on code.google.com by `czarek.t...@gmail.com` on 11 Apr 2014 at 5:39
