Validate Grouping while validating GroupAssignment

[EricPickup]

Fixes #2230.

Problem

Groupings are persisted when their associating GroupAssignment is saved, however, if the grouping fails its validation the group assignment will still save with a nil grouping_id.

Solution

Use the validates_associated helper on GroupAssignment to validate the grouping as part of the assignment's validation. I've also added a couple methods to fetch the errors straight from the grouping object since validates_associated only adds a generic Your grouping is invalid error message on the group assignment object.