Improved handling for validation in field groups

[Shadowfiend]

Currently field groups have withBoxedConverter, which can return a non-Full Box to indicate validation failures. There are, however, a couple of downsides to this approach:

• The resulting failures have to be manually handled in onFailure, or the withBoxedConverter function has to explicitly invoke S.error to mark the client error.
• The converter may produce an object of type T that is usually valid but isn't valid in the context of this form. This T may need further validation in this particular context.

Two things would help here:

• Alongside Custom validation attachment points #15, we could define validations with custom attachment points on the post-converted T type.
• Alongside Pushing validation side-effects later? #16, we could return a Failure of the same type that a field would, including attachment point information and error message information.