skimage: report failure if expectation not found.

If the expectation file exists, but the particular file being
decoded has no expectation, report a failure.

This is more appropriate since the intended use is that an
expectation will be created for each file. Without an expectation,
we cannot know whether the decoder has changed.

R=epoger@google.com

Review URL: https://codereview.chromium.org/15094011

git-svn-id: http://skia.googlecode.com/svn/trunk@9122 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed