Serve an image on error

I broke images yesterday (deleted the cache directory in addition to its contents when I tried to free up some disk space on the server). They were returning 500 which was probably confusing for any place that they are embedded.

We should catch errors thrown in the image generation path and return a special 500 image if it fails.

Start at the image generation path and trace how errors are currently returned to the requester. The work is done when generation failures return a special 500 image instead of an unhandled 500 response, including failures caused by a missing cache directory.

Written by the indexing model from the issue text.