delete PowerPoint presentation before removing from service

[John Cegalis]

Win 7 nightly build 2523.

Add .ppt or .pptx file to Media Manager and then close OpenLP. Delete or rename the .ppt. pptx file that you are using (I suggest rename). Try to open OpenLP and get this traceback. There is nothing you can do but acknowledge the traceback and the program closes. You do not have a chance to remove the offending file from OpenLP to stop the traceback. If you deleted the file, you have to recreate it with the same name to get OpenLP working again.

Traceback (most recent call last):
  File "<string>", line 44, in <module>
  File "D:\OpenLP_Development\OpenLP_Code\openlp\openlp\core\__init__.py", line 377, in main
  File "D:\OpenLP_Development\OpenLP_Code\openlp\openlp\core\__init__.py", line 161, in run
  File "D:\OpenLP_Development\OpenLP_Code\openlp\openlp\core\ui\mainwindow.py", line 650, in app_startup
  File "C:\PROGRA~2\OpenLP\plugins\presentations\presentationplugin.py", line 147, in app_startup
    self.media_item.clean_up_thumbnails(file, True)
  File "D:\OpenLP_Development\OpenLP_Code\openlp\openlp\plugins\presentations\lib\mediaitem.py", line 248, in clean_up_thumbnails
  File "c:\Python33\lib\genericpath.py", line 54, in getmtime
FileNotFoundError: [WinError 2] The system cannot find the file specified: 'C:\\Users\\Grace2\\Desktop\\Grace Church Sunday Morning.pptx'