Unmarshals an ACK from an acknowledgement returned by the SDK. If
the returned acknowledgement can not be parsed into an ACK,
err(base64(ack)) is returned.
wasmd 0.32+ will not return a hardcoded ICS-20 ACK if
ibc_packet_receive errors 1 so we can safely use an ACK format
that is not ICS-20 error-variant compatible.