Make media assets show a placeholder thumbnail when the image is
missing. This can happen if the upload is still processing, or if the
media asset's image was expunged, or if the asset failed during upload
(usually because of some temporary network failure when trying to
distribute thumbnails to the backend image servers).
Fixes a problem where new images on the My Uploads or All Uploads pages
could have broken thumbnails if they were still in the uploading phase.
