field validation errors not always formatted properly

[macie-korte]

Error strings with format markers do not always have the proper variables substituted in in the errors dictionary.

For example, this validation error message comes back from a ChoiceField after serializer validation is done. From example_serializer.errors:

  {
    "some_type": [
      "Select a valid choice. %(value)s is not one of the available choices."
    ]
  }