Do not give up if there are still eligible thumbnailers for a pair (mime type, uri)
Currently, Tumbler forwards requests it receives to the highest priority thumbnailer for a pair (mime type, uri), and considers it failed if that thumbnailer failed.
I don't see a good reason for this. From the outside perspective, it doesn't matter whether one thumbnailer or another handles a request, only whether a thumbnail is produced or not. And while it is possible to set the priority of thumbnailers, priority does not mean exclusivity.
So if the first thumbnailer fails, Tumbler should forward requests to lower priority thumbnailers if any, and only consider itself to be failing if all eligible thumbnailers have failed.
This should be done keeping in mind #49 (closed): the final error in case of failure must fit within the spec, and the treatment of intermediate errors is still to be determined (whether there is a final success or failure). An extra information reserved for the debug mode is possible.