Fixed #16192 -- Made unique error messages in ModelForm customizable.

Overriding the error messages now works for both unique fields, unique_together
and unique_for_date.

This patch changed the overriding logic to allow customizing NON_FIELD_ERRORS
since previously only fields' errors were customizable.

Refs #20199.

Thanks leahculver for the suggestion.

docs/ref/exceptions.txt

@@ -124,6 +124,15 @@ ValidationError
     :ref:`Model Field Validation <validating-objects>` and the
     :doc:`Validator Reference </ref/validators>`.
 
+NON_FIELD_ERRORS
+~~~~~~~~~~~~~~~~
+.. data:: NON_FIELD_ERRORS
+
+``ValidationError``\s that don't belong to a particular field in a form
+or model are classified as ``NON_FIELD_ERRORS``. This constant is used
+as a key in dictonaries that otherwise map fields to their respective
+list of errors.
+
 .. currentmodule:: django.core.urlresolvers
 
 URL Resolver exceptions

docs/ref/models/fields.txt

@@ -233,8 +233,12 @@ field will raise. Pass in a dictionary with keys matching the error messages you
 want to override.
 
 Error message keys include ``null``, ``blank``, ``invalid``, ``invalid_choice``,
-and ``unique``. Additional error message keys are specified for each field in
-the `Field types`_ section below.
+``unique``, and ``unique_for_date``. Additional error message keys are
+specified for each field in the `Field types`_ section below.
+
+.. versionadded:: 1.7
+
+The ``unique_for_date`` error message key was added.
 
 ``help_text``
 -------------

docs/ref/models/instances.txt

@@ -151,8 +151,8 @@ access to more than a single field::
 
 Any :exc:`~django.core.exceptions.ValidationError` exceptions raised by
 ``Model.clean()`` will be stored in a special key error dictionary key,
-``NON_FIELD_ERRORS``, that is used for errors that are tied to the entire model
-instead of to a specific field::
+:data:`~django.core.exceptions.NON_FIELD_ERRORS`, that is used for errors
+that are tied to the entire model instead of to a specific field::
 
     from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
     try:

docs/ref/models/options.txt

@@ -321,6 +321,11 @@ Django quotes column and table names behind the scenes.
     :class:`~django.db.models.ManyToManyField`, try using a signal or
     an explicit :attr:`through <ManyToManyField.through>` model.
 
+    .. versionchanged:: 1.7
+
+    The ``ValidationError`` raised during model validation when the
+    constraint is violated has the ``unique_together`` error code.
+
 ``index_together``
 ------------------