allow greater control over which fields' errors trigger error styling

Addresses the problem explained in #42.

As a manually-initiated group has no way of knowing what is going to be put in that group when its opened, it doesn't have a clue as to whether those unknown field have associated validation errors. The current behaviour - doing what it did for the last field of the last group isn't very satisfactory.

This pull request allow the developer inserting the group by hand, to specify what fields should be tested against when determining whether to apply error styling.

This isn't the same as specifying what fields will be in the group. When fields are grouped they may be validated together to reflect interactions between them. This change allows the results of validation against all, some or none of the fields to be used to determine whether to apply error styling to the group.