Create our error encrypter with a wrapped type if we have a blinding
point present. Doing this in the iterator allows us to track this
information when we have both pieces of information available to us,
compared to trying to handle this later down the line:
- Downstream link on failure: we know that we've set a blinding point
for out outgoing HTLC, but not whether we're introduction or not
- Upstream link on failure: once the failure packet has been sent
through the switch, we no longer know whether we were the introduction
point (without looking it up / examining our payload again /
propagating this information through the switch).
72260adddb