[dasc] Field validation UI

As an author, I would like to know if my object, array, or simpleton is not valid.

## Criteria of acceptance
1. Simpleton's show errors below the field.
2. Objects highlight red (soft red background?)
3. Objects have human-readable text stating the issue
4. Arrays highlight red (soft red background?)
5. Arrays have a human-readable text stating the issue
6. Uses existing FormModel validation function.
7. Any form with errors should have _some kind_ of warning.

## Out of scope
1. Showing any errors in the outline - I'm not convinced this is needed. We can revisit.

Start with the existing FormModel validation function and trace how simpletons, objects, arrays, and forms are rendered. Implement visible field or container warnings and human-readable messages for invalid values, then verify that every form with errors shows a warning while outline errors remain out of scope.

Written by the indexing model from the issue text.