Temp deletion

@drsnyder 
Instead of leaving dangling files on the filesystem after a thumb is created, delete it after the stream is closed. I think we'll have to modify the purge cron to send error output to /dev/null, since this should amplify the `find: <filename>: No such file or directory` race condition.

Also piggybacking in the ability to send a custom error response code when using throw+ to the general catch statement
Could this be simplified to:

```clojure
(close []
   (proxy-super close)
   (map #(% stored-object) (:stream-close-callbacks stored-object)))
```