Handle OSError exceptions on tile sources.
Handle OSError exceptions on tile sources.

When a tile source fails to import due to an OSError, treat it much like an ImportError.  This is the error we get when a library is missing but the python module is present.

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