Add FailureCode enum and ChannelManager::fail_htlc_backwards_with_reason
FailureCode is used to specify which error code and data to send
to peers when failing back an HTLC.
ChannelManager::fail_htlc_backwards_with_reason
allows a user to specify the error code and
corresponding data to send to peers when failing back an HTLC.
This function is mentioned in Event::PaymentClaimable docs.
ChannelManager::get_htlc_fail_reason_from_failure_code was also
added to assist with this function.