Set an error code for phone_number_validator.

`phone_number_validator` works well when validating from a model, but not
so well when validating from a form field. It uses the default code of
`invalid`, which the Django fields will map to a pre-defined error
message, ignoring the custom one given by the validator.

By setting a custom error code, this mapping won't happen, and the
validator's message will instead be used.