glance should fail more gracefully on requests for images that have been removed from the fs

If for whatever reason an image file is removed from the filesystem and the entry for it still exists in the database, glance should fail more gracefully. Right now, if you try to do a HEAD request for the image, you get a valid response. That's fine. But then when you do a 'GET /image/<id>', glance just closes the socket connection. Seems like a 404 or something should be returned, instead.
I'm guessing this bug might affect where permissions on the image file have changed such that glance can't open it.. as well. But I've not attempted to test that case.