Fail channel when BADONION is not set

[yuntai]

when update_fail_malformed_htlc is received.

if the BADONION bit in failure_code is not set for update_fail_malformed_htlc:
MUST fail the channel.

found while testing for onion_error handling (#146, which is close to being finished!)

There is another condition in the spec

if the sha256_of_onion in update_fail_malformed_htlc doesn't match the onion it sent:
MAY retry or choose an alternate error response.
, which is not clear as to what 'alternate error response' should be used. And `retry' for random bit error?