IPAddressField does not fully respect protocol='IPv6'

[Heinibal]

Problem Description

I am using the IPAddressField to validate ipv6 values and noticed that the error message for invalid values sometimes is 'Enter a valid IPv4 or IPv6 address.' instead of the more appropriate 'Enter a valid IPv6 address.'
The latter message appears if the invalid value doesn't contain the character ':'. The reason is that djangos clean_ipv6_address method is only called by IPAddressField.to_internal_value if that character appears in the data:

        if ':' in data:
            try:
                if self.protocol in ('both', 'ipv6'):
                    return clean_ipv6_address(data, self.unpack_ipv4)
            except DjangoValidationError:
                self.fail('invalid', value=data)

Suggested Solution

IPAddressField.__init__ already retrieves the appropriate error message for invalid values but does not actually use it. My suggestion would be to update the fields error_messages:

    def __init__(self, protocol='both', **kwargs):
        self.protocol = protocol.lower()
        self.unpack_ipv4 = (self.protocol == 'both')
        super().__init__(**kwargs)
        validators, error_message = ip_address_validators(protocol, self.unpack_ipv4)
+       self.error_messages.update({'invalid': error_message})
        self.validators.extend(validators)

I would be happy to work on this and create a pull request if this is determined to be a valid improvement. Of course I am also open to suggestions how to better fix this issue.