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.")}}
```
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