Pushing validation side-effects later?

[Shadowfiend]

Currently validation side-effects are done in Field; that is, S.error is invoked during field handling. It may be worth considering pushing that into the form, so that the field simply returns a Box potentially including its error data, and the form submission handler is charged with invoking S.error. This would make it easier for the invoker to change the behavior (e.g., by not invoking S.error but marking the errors on the client differently, for example). It would also separate concerns a bit better, allowing the field to be solely charged with coordinating the field data, and not with marking the validation error in a client-specific way.