This commit adds a hop.PayloadViolation enum which encompasses the cases
where the sender omits, includes, or requires a type that causes an
ErrInvalidPayload faiulre.
The existing Omitted bool is converted to this PayloadViolation, and
NewPayloadFromReader is updated to return such a failure with a
RequiredViolation when an unknown required type is detected.
The unit tests are updated to cover the three possible cases of
RequiredViolations, as well as included valid intermediate and final hop
tests.
From BOLT 04:
The writer:
- MUST include amt_to_forward and outgoing_cltv_value for every node.
- MUST include short_channel_id for every non-final node.
- MUST NOT include short_channel_id for the final node.
