Raise ValidationError if the address is invalid. Replace the longest continuous zero-sequence with "::", remove leading zeroes, and make sure all hextets are lowercase. Args: ip_str: A valid IPv6 address. unpack_ipv4: if an IPv4-mapped address is found, return the plain IPv4 address (default=False). error_message: An error message used in the ValidationError. Return a compressed IPv6 address or the same value.