Handle KeyError when trying to get openslide properties.
Handle KeyError when trying to get openslide properties.

This change allows a few more files to be opened by the svs tilesource.
For instance, if you import the .mrxs named file from a group of mrxs
files, this will allow them to be displayed.

girder/large_image,DigitalSlideArchive/large_image,DigitalSlideArchive/large_image,girder/large_image,DigitalSlideArchive/large_image,girder/large_image