Reftest result viewer is confusing with a mismatch test

[gsnedders]

e.g. https://wpt.fyi/analyzer?screenshot=sha1%3Ac06f20c60c4978a2e283980f57871050f9a765af&screenshot=sha1%3Ac06f20c60c4978a2e283980f57871050f9a765af

This shows, for a failure, "Actual screenshot" and "Expected screenshot" with a "maxDifference: 0".

This is confusing, until I realised that it was probably a mismatch test, and looked, and lo, it is. We should make the viewer clearer when it's a mismatch.