Print the error when image_diff fails when comparing reftests.

Before this patch, we'd say that the image diff passed.

Review URL: https://chromiumcodereview.appspot.com/23272007

git-svn-id: svn://svn.chromium.org/blink/trunk@156754 bbb929c8-8fbe-4397-9dbb-9b2b20218538