title: pyscreenshot get remote windows server's screenshot without login
body: <p>The python2 script was executed by the system daemon process(such like SaltStack, the process user is Administrator), the script worked well and saved the screenshot picture while I was connected and login the server, but it will raise Error without someone has been login.</p>
  
  <pre><code>import pyscreenshot
  getPic = pyscreenshot.grab(bbox=(0, 0, 1920, 1080)) ## line 43
  # getPic = pyscreenshot.grab() ## work as same as before line
  getPic.save('screenshot.png')
  </code></pre>
  
  <p>Error info:</p>
  
  <pre><code>Traceback (most recent call last):
    File ".../.../get_desktop_screenshot.py", line 43, in &lt;module&gt;
    File "C:\Python27\lib\site-packages\pyscreenshot\__init__.py", line 43, in grab
      return _grab(childprocess=childprocess, backend=backend, bbox=bbox)
    File "C:\Python27\lib\site-packages\pyscreenshot\__init__.py", line 29, in _grab
      return childprocess_grab(_grab_simple, backend, bbox)
    File "C:\Python27\lib\site-packages\pyscreenshot\childproc.py", line 34, in childprocess_grab
      return childprocess_grab_popen(backend, bbox)
    File "C:\Python27\lib\site-packages\pyscreenshot\childproc.py", line 54, in childprocess_grab_popen
      raise FailedBackendError(p)
  pyscreenshot.loader.FailedBackendError: &lt;EasyProcess cmd_param=['C:\\Python27\\python.exe', '-m', 'pyscreenshot.cli.grab_to_file', 'c:\\users\\admini~1\\appdata\\local\\temp\\pyscreenshothgubzc\\screenshot.png', '0', '0', '1920', '1080', '--backend', ''] cmd=['C:\\Python27\\python.exe', '-m', 'pyscreenshot.cli.grab_to_file', 'c:\\users\\admini~1\\appdata\\local\\temp\\pyscreenshothgubzc\\screenshot.png', '0', '0', '1920', '1080', '--backend', ''] oserror=None return_code=1 stdout="" stderr="Traceback (most recent call last):
  
    File "C:\Python27\lib\runpy.py", line 174, in _run_module_as_main
      "__main__", fname, loader, pkg_name)
    File "C:\Python27\lib\runpy.py", line 72, in _run_code
      exec code in run_globals
    File "C:\Python27\lib\site-packages\pyscreenshot\cli\grab_to_file.py", line 8, in &lt;module&gt;
      def main(filename, x1, y1, x2, y2, backend=""):
    File "C:\Python27\lib\site-packages\entrypoint2\__init__.py", line 440, in entrypoint
      return func(**kwargs)
    File "C:\Python27\lib\site-packages\pyscreenshot\cli\grab_to_file.py", line 28, in main
      im = pyscreenshot.grab(bbox=bbox, childprocess=False, backend=backend)
    File "C:\Python27\lib\site-packages\pyscreenshot\__init__.py", line 43, in grab
      return _grab(childprocess=childprocess, backend=backend, bbox=bbox)
    File "C:\Python27\lib\site-packages\pyscreenshot\__init__.py", line 31, in _grab
      return _grab_simple(backend, bbox, filename)
    File "C:\Python27\lib\site-packages\pyscreenshot\__init__.py", line 17, in _grab_simple
      return backend_obj.grab(bbox)
    File "C:\Python27\lib\site-packages\pyscreenshot\plugins\pil.py", line 17, in grab
      return self.ImageGrab.grab(bbox)
    File "C:\Python27\lib\site-packages\PIL\ImageGrab.py", line 41, in grab
      size, data = grabber()
  IOError: screen grab failed" timeout_happened=False&gt;
  </code></pre>