These use onion encoding for simple one-way messaging: there are no error returns.
However, every onion uses route blinding *even if it doesn't need to*.
You can prove what path was used to reach you by including `path_id` in the
encrypted_data_tlv.
Note that this doesn't actually define the payload we're transporting:
that's explictly defined to be payloads in the 64-255 range.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>