17219 fix custom validator display if function

[arthanson]

Fixes: #17219

Tested with function custom validator:

from extras.validators import CustomValidator

class UniqueSerial(CustomValidator):

    def validate(self, instance):
        from dcim.models import Device

        if instance.serial and Device.objects.exclude(pk=instance.pk).filter(serial=instance.serial).exists():
            self.fail("The Serial number already exists in NetBox", field='serial')

CUSTOM_VALIDATORS = {
    'dcim.device': (
        UniqueSerial(),
    )
}

and with dictionary based:

CUSTOM_VALIDATORS = {
    "dcim.device": [
        {
            "name": {
                "regex": "[a-z]+\d{3}"
            },
            "asset_tag": {
                "required": True
            }
        }
    ]
}

[jeremystretch]

This fails when configuring both class- and dictionary-based validators:

CUSTOM_VALIDATORS = {
    'dcim.device': [
        {
            "name": {
                "min_length": 5
            }
        },
        UniqueSerial(),
    ]
}

[jeremystretch]

It might be cleaner to just write a custom JSON encoder (like CustomFieldJSONEncoder) to serialize Python classes to their string names. We can then do the serialization inside the view (rather than the template) and pass the rendered JSON directly as context data for the template.