Error handling of tile image loading in PanoJS See #11709

This has not yet been merged into develop #1818 but I'm back-porting to make testing easier, and it might address a critical issue in dev_4_4. E.g. see https://nightshade.openmicroscopy.org/webgateway/img_detail/3946831/

For testing and more info see #1818

--rebased-from #1818