protect feeder from corrupted images

Hall C had an image that feeder tried to analyze but couldn't based on the following error:

libpng error: IDAT: CRC error

which caused feeder to get "stuck" and continually try to process an image it couldn't. a similar issue appeared earlier with symlink-ed images in gluex a couple of months back. the symptoms of this are the page not updating on the front end, and on the back end feeder's log file grows.