Correctly wrap phantom onion errors

Correctly wrap phantom onion errors

In any place where fail_htlc_backwards_internal was called for a phantom payment
failure, we weren't encoding the onion failure as if the phantom were the one
failing. Instead, we were encoding the failure as if it were coming from the
second-to-last hop. This caused our failures to not be parsed properly on the
payer's side.

Places we were encoding failures incorrectly include:
* on failure of a call to inbound_payment::verify
* on a user call to fail_htlc_backwards

Also drop some unnecessary panics when reading OnionHopData objects. This also
enables one of the phantom failure tests because we can construct OnionHopDatas
with invalid amounts.

Lastly, remove a bogus comment