Handle custom field validation errors without validate methods

[gaoflow]

Summary

• guard lookup of serializer-level validate_ methods when classifying field errors
• keep existing fallback behavior for unknown custom field errors instead of raising AttributeError
• add a regression test for ValidationError raised from a custom Field.to_internal_value()

Fixes #8.

Tests

• uv run --python 3.10 --with 'django<3' --with 'djangorestframework<3.12' --with pytest --with pytest-django --with six --with-editable . pytest tests/test_serializers.py::SerializerErrorsTestCase::test_custom_field_validation_error_without_validate_method -q
• uv run --python 3.10 --with 'django<3' --with 'djangorestframework<3.12' --with pytest --with pytest-django --with six --with-editable . pytest tests/test_serializers.py -q
• uv run --python 3.10 --with 'django<3' --with 'djangorestframework<3.12' --with pytest --with pytest-django --with six --with 'flake8==2.4.0' --with 'pep8==1.5.7' --with-editable . python runtests.py --lintonly
• git diff --check

Note: I also tried the full test suite. With Django 2 it fails during collection because the existing tests import django.core.urlresolvers. With Django 1.11 and older DRF on Python 3.9 it collects but has existing ModelSerializer compatibility failures unrelated to this change.