Wagtail NonFieldErrors for Blocks

blocks.py

class NonFieldErrors:
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.non_field_errors = []
        self.meta.form_template = 'struct.html'

    def get_form_context(self, value, prefix='', errors=None):
        context = super().get_form_context(value, prefix, errors)
        context['non_field_errors'] = self.non_field_errors
        return context

    def add_non_field_errors(self, errs: List[str]) -> None:
        self.non_field_errors = errs
        raise ValidationError("The page could not be saved due to validation errors", params={
            NON_FIELD_ERRORS: ErrorList(errs)
        })
        

class Person(NonFieldErrors, blocks.StructBlock):
    name = blocks.CharField()
    age = blocks.IntegerField()

    def clean(self, value)
        value = super().clean(value)
        if value["name"] == "Joe Bloggs" and value["age"] >= 100:
                self.add_non_field_errors(["I don't believe you."])
        return value

struct.html

{# Original file: https://github.com/wagtail/wagtail/blob/master/wagtail/admin/templates/wagtailadmin/block_forms/struct.html #}

<div class="{{ classname }}">
  {% if non_field_errors %}
  <div class="messages">
    <ul>
      {% for err in non_field_errors %}
        <li class="error">{{ err }}</li>
      {% endfor %}
    </ul>
  </div>
  <br>
  {% endif %}

  {% if help_text %}
    <div class="object-help help">{{ help_text }}</div>
  {% endif %}

  <ul class="fields">
    {% for child in children.values %}
      <li{% if child.block.required %} class="required"{% endif %}>
        {% if child.block.label %}
          <label{% if child.id_for_label %} for="{{ child.id_for_label }}"{% endif %}>{{ child.block.label }}:</label>
        {% endif %}
        {{ child.render_form }}
      </li>
    {% endfor %}
  </ul>
</div>