This commit does away with the ErrorCode field in order to ensure that
we’re properly compatible with BOLT-0002. In the future the error code
field may be re-introduced as it’s much easier to check against an
internet error rather than interpret a byte slice describing the error.
This commit modifies the Message interface to convert the Command
method to a MsgType method that uses a new set of message type for all
the defined messages. These new messages types nearly exactly match the
message types used within the current draft of the BOLT specifications.
This commit revues the Validate method from the Message interface as
the method is no longer used and is a relic from an older version of
the codebase.
