Show human-readable label in DAD form validation errors

[jeancochrane]

The DAD form displays errors with the following code:

{% for field, errors in form.errors.items %}
  <li>{{field}}: {{errors}}</li>
{% endfor %}

Unfortunately, field displays the raw field name, not the human-readable label:

Once we have reliable errors that we can return, figure out how to display the human-readable label instead of the raw field name in errors.