When an object fails the jp2-create step, clear the temp tiff from the imtmp directory

[blalbrit]

We've noticed that, for objects that fail the jp2-create step, a temporary tiff (produced by imagemagick) remains in the /imtmp folder. Eventually, this impinges on available disk space and the stale tiffs must be manually cleared.

Suggest a periodic sweep of this directory for files older than some timestamp (several hours?).

@LynnMcRae - what do you think?