Always treat invalid images as nonexistant now.

This makes the sequence of pictures shown to the user slightly less
confusing, but the whole process needs a bit more cleanup. Some of the work
has to be done in the BIOS code, not just here. This is better, but it's not
complete.

Change-Id: If0b3ecc92716020a06031866849d83411d7f0db8

BUG=chrome-os-partner:903
TEST=manual

Review URL: http://codereview.chromium.org/3249009
2 files changed