There are three cases:
1. failcode is 0, scid is NULL, failreason is the onion to fwd.
2. failcode is non-zero, but UPDATE bit not set. scid is NULL, failreason NULL.
3. failcode has UPDATE bit set. scid is non-NULL, failreason is NULL.
Assert these on marshaling, and only send the parts we need so unmarshal is
always canonical.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>