This commit adds handling for malformed HTLC errors related to blinded
paths. We expect to receive these errors _within_ a blinded path,
because all non-introduction nodes are instructed to return malformed
errors for failures.
Note that we may actually switch back to a malformed error later on if
we too are a relaying node in the route, but we handle that case the
incoming link.
Carla Kirk-Cohen
c2c0158c84