Grid components cannot be programmatically partially required

An unanticipated consequence of providing proper server-side validation of grid components (see related issue) is that custom modules can no longer mark some questions in a required grid as not being required. Previously a form after_build function could remove the required attribute from some of the radio button(s), allowing those questions within the grid to be optional. The browser respects this but then the server now enforces validation.

The proposed solution is to re-check that each question is still required before generating a validation error for the required attribute.