Fixed #25241 -- Corrected ModelForm.save() error message when saving invalid form with UUIDField pk.

tests/model_forms/test_uuid.py

@@ -0,0 +1,29 @@
+from __future__ import unicode_literals
+
+from django import forms
+from django.test import TestCase
+
+from .models import UUIDPK
+
+
+class UUIDPKForm(forms.ModelForm):
+    class Meta:
+        model = UUIDPK
+        fields = '__all__'
+
+
+class ModelFormBaseTest(TestCase):
+    def test_create_save_error(self):
+        form = UUIDPKForm({})
+        self.assertFalse(form.is_valid())
+        msg = "The UUIDPK could not be created because the data didn't validate."
+        with self.assertRaisesMessage(ValueError, msg):
+            form.save()
+
+    def test_update_save_error(self):
+        obj = UUIDPK.objects.create(name='foo')
+        form = UUIDPKForm({}, instance=obj)
+        self.assertFalse(form.is_valid())
+        msg = "The UUIDPK could not be changed because the data didn't validate."
+        with self.assertRaisesMessage(ValueError, msg):
+            form.save()