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.