This commit adds a new field to the CancelHTLC message which describes
the event that led to an HTLC being cancelled up stream.
A new enum has been added which describers the “why” concerning the
cancellation of the HTLC. Currently the encoding and back propagation
of the errors aren’t properly implemented as defined within the spec.
As a result the current error types provide to privacy as the error are
in plain-site rather doing being properly encrypted.
This commit removes the previous HTLC timeout in message in favor of a
HTLC cancel message. Within the protocol, a timeout message would never
be sent backwards along the route as in the case of an HTLC timeout
before/after the grace period, the course of action taken would be to
broadcast the current commitment transaction unilaterally on-chain.