OpenImageIO/oiio — pull request #564 by lgritz (2013-04-05 00:19:59+00:00)
Title: Better error handling for oiiotool --diff.

Botched the last error handling patch -- this is what I wanted.
The difference is that we want a "failed to match" to output the difference
image, if that is requested. But a "failed to diff" (e.g. images have
differing number of channels) should be a real failure and abort.