[1.7.x] Fixed #24008 -- Fixed ValidationError crash with list of dicts.

Backport of 7a878ca5cb from master

django/core/exceptions.py

@@ -118,7 +118,10 @@ class ValidationError(Exception):
                 # Normalize plain strings to instances of ValidationError.
                 if not isinstance(message, ValidationError):
                     message = ValidationError(message)
-                self.error_list.extend(message.error_list)
+                if hasattr(message, 'error_dict'):
+                    self.error_list.extend(sum(message.error_dict.values(), []))
+                else:
+                    self.error_list.extend(message.error_list)
 
         else:
             self.message = message

docs/releases/1.7.2.txt

@@ -186,3 +186,6 @@ Bugfixes
 
 * Restored the ability to use more than five levels of subqueries
   (:ticket:`23758`).
+
+* Fixed crash when ``ValidationError`` is initialized with a ``ValidationError``
+  that is initialized with a dictionary (:ticket:`24008`).

tests/forms_tests/tests/test_utils.py

@@ -45,9 +45,31 @@ class FormsUtilsTestCase(TestCase):
         self.assertHTMLEqual(str(ErrorList(ValidationError(["Error one.", "Error two."]).messages)),
                          '<ul class="errorlist"><li>Error one.</li><li>Error two.</li></ul>')
 
+        # Can take a dict.
+        self.assertHTMLEqual(
+            str(ErrorList(sorted(ValidationError({'error_1': "1. Error one.", 'error_2': "2. Error two."}).messages))),
+            '<ul class="errorlist"><li>1. Error one.</li><li>2. Error two.</li></ul>'
+        )
+
         # Can take a mixture in a list.
-        self.assertHTMLEqual(str(ErrorList(ValidationError(["First error.", "Not \u03C0.", ugettext_lazy("Error.")]).messages)),
-                         '<ul class="errorlist"><li>First error.</li><li>Not π.</li><li>Error.</li></ul>')
+        self.assertHTMLEqual(
+            str(ErrorList(sorted(ValidationError([
+                "1. First error.",
+                "2. Not \u03C0.",
+                ugettext_lazy("3. Error."),
+                {
+                    'error_1': "4. First dict error.",
+                    'error_2': "5. Second dict error.",
+                },
+            ]).messages))),
+            '<ul class="errorlist">'
+            '<li>1. First error.</li>'
+            '<li>2. Not π.</li>'
+            '<li>3. Error.</li>'
+            '<li>4. First dict error.</li>'
+            '<li>5. Second dict error.</li>'
+            '</ul>'
+        )
 
         @python_2_unicode_compatible
         class VeryBadError: