Before this change we have a mish-mash of ways to pass field names around for
error generation.  Sometimes string fieldnames, sometimes .Prefix(), sometimes
neither, often wrong names or not indexed when it should be.

Instead of that mess, this is part one of a couple of commits that will make it
more strongly typed and hopefully encourage correct behavior.  At least you
will have to think about field names, which is better than nothing.

It turned out to be really hard to do this incrementally.