BUG: filesize_unlimited() did not catch COM exceptions.

* Thanks for the hint, Neil!
* Slight optimization in get_image_sizes() to only get the file size if
  the file in question actually exists.


git-svn-id: http://svn.montala.net/svn/resourcespace@5084 c08608d7-6e46-0410-86ca-f2a6f1370df5