Handle exceptions in ClaimDifferenceVisualizer

@note: we'll want a nicer solution that is more consistent
and shares code as how we handle things in SnakHtmlGenerator.

But this is an improvement over outputting '?' in a diff.

Bug: 63710
Change-Id: I683f086b561b2d1d077a1ae919fa6c4f9d507610
Merge "Handle exceptions in ClaimDifferenceVisualizer"