Fixed #21654 -- Documented a use-case for Form.errors.as_data().

    {'sender': [ValidationError(['Enter a valid email address.'])],

    'subject': [ValidationError(['This field is required.'])]}

Use this method anytime you need to identify an error by its ``code``. This

enables things like rewriting the error's message or writing custom logic in a

view when a given error is present. It can also be used to serialize the errors

in a custom format (e.g. XML); for instance, :meth:`~Form.errors.as_json()`

relies on ``as_data()``.

The need for the ``as_data()`` method is due to backwards compatibility.

Previously ``ValidationError`` instances were lost as soon as their

**rendered** error messages were added to the ``Form.errors`` dictionary.

Ideally ``Form.errors`` would have stored ``ValidationError`` instances

and methods with an ``as_`` prefix could render them, but it had to be done

the other way around in order not to break code that expects rendered error

messages in ``Form.errors``.

.. method:: Form.errors.as_json()

.. versionadded:: 1.7

    ValidationError(_('Invalid value: %s') % value)

.. versionadded:: 1.7

The :meth:`Form.errors.as_data() <django.forms.Form.errors.as_data()>` and

:meth:`Form.errors.as_json() <django.forms.Form.errors.as_json()>` methods

greatly benefit from fully featured ``ValidationError``\s (with a ``code`` name

and a ``params`` dictionary).

Raising multiple errors

~~~~~~~~~~~~~~~~~~~~~~~