This causes the thumbnailing thread pool to spend a lot of time attempting to
retry an image that failed. A null result is common where the image is too large
to be allocated (>128MiB alloc).
The repeated retries continue after page delete, causing hangs if a user
tries to exit the application.
Fixes: #1201
Signed-off-by: James Beddek <telans@posteo.de>
75bd626f33