Handle Django's ValidationErrors in ListField (#6423)

Without this, Django's ValidationErrors will bypass the error collection
from ListField's children.

Here is an example that illustrates this change.

Consider a Serializer that uses ListField like this:

```python
class SomeSerializer(serializers.Serializer):
    uuids = serializers.ListField(
	child=serializers.PrimaryKeyRelatedField(
	    queryset=Model.objects.something(),
	    validators=[SomeCustomValidator()]
	)
    )
```

Validating data that looks like this works fine:

```python
{uuids: ['some-valid-uuid', 'some-valid-uuid']}
```

Raising a DRF ValidationError for one of the children works fine, giving
an error object like:

```python
{'uuids': {0: ErrorDetail(string='Some validation error')}}
```

Raising a Django ValidationError for one of the children works
differently (which serializers.PrimaryKeyRelatedField can do in some
cases, like when the uuid is malformed). It gives an error object like:

```python
{'uuids': ["'X' is not a valid UUID."]}
```

Handling Django's ValidationErrors in ListField explicitly (like in this
pull request), will maintain a regular error interface in this case:

```python
{'uuids': {0: ErrorDetail(string="'X' is not a valid UUID.")}}
```

rest_framework/fields.py

@@ -1639,6 +1639,8 @@ class ListField(Field):
                 result.append(self.child.run_validation(item))
             except ValidationError as e:
                 errors[idx] = e.detail
+            except DjangoValidationError as e:
+                errors[idx] = get_error_detail(e)
 
         if not errors:
             return result

tests/test_fields.py

@@ -17,6 +17,7 @@ from rest_framework import exceptions, serializers
 from rest_framework.fields import (
     BuiltinSignatureError, DjangoImageField, is_simple_callable
 )
+from tests.models import UUIDForeignKeyTarget
 
 utc = datetime.timezone.utc
 
@@ -2074,6 +2075,35 @@ class TestNestedListField(FieldValues):
     field = serializers.ListField(child=serializers.ListField(child=serializers.IntegerField()))
 
 
+class TestListFieldWithDjangoValidationErrors(FieldValues, TestCase):
+    """
+    Values for `ListField` with UUIDField as child
+    (since UUIDField can throw ValidationErrors from Django).
+    The idea is to test that Django's ValidationErrors raised
+    from Django internals are caught and serializers in a way
+    that is structurally consistent with DRF's ValidationErrors.
+    """
+
+    valid_inputs = []
+    invalid_inputs = [
+        (
+            ['not-a-valid-uuid', 'd7364368-d1b3-4455-aaa3-56439b460ca2', 'some-other-invalid-uuid'],
+            {
+                0: [exceptions.ErrorDetail(string='“not-a-valid-uuid” is not a valid UUID.', code='invalid')],
+                1: [
+                    exceptions.ErrorDetail(
+                        string='Invalid pk "d7364368-d1b3-4455-aaa3-56439b460ca2" - object does not exist.',
+                        code='does_not_exist',
+                    )
+                ],
+                2: [exceptions.ErrorDetail(string='“some-other-invalid-uuid” is not a valid UUID.', code='invalid')],
+            },
+        ),
+    ]
+    outputs = {}
+    field = serializers.ListField(child=serializers.PrimaryKeyRelatedField(queryset=UUIDForeignKeyTarget.objects.all()))
+
+
 class TestEmptyListField(FieldValues):
     """
     Values for `ListField` with allow_empty=False flag.