From review of the style with @carljbowman
Requires a bit of magic, where the Form component manually adds the "all" value. But without a major overhaul of the validation system, I don't see how else to put the general error among the other fields.
I don't like that the state is being set to a calculation of existing props. When I did that calculation in `render`, it didn't update when fields were changed.
Fixes#645